Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

- Use `cover' with QuickCheck testing
- Reuse the `cover_*' rebar.config options
- Refactor cover-related code to separate module (`qc_cover_utils')
  for use with both `eunit' and `qc'

Always-on recursive application of all rebar commands causes too many
issues. Recursive application is required for:
1. dealing with dependencies: get-deps, update-deps, and compile of deps
   right after get-deps or update-deps
2. projects with a riak-like apps/ project structure and dev process

The vast majority of projects are not structured like riak. Therefore,
moving forward it's best to (by default) restrict recursive behavior to
dealing with deps. This commit does that and also adds command line and
rebar.config options for controlling or configuring recursion. Also, we
introduce two meta commands: prepare-deps (equivalent to rebar -r
get-deps compile) and refresh-deps (equivalent to rebar -r update-deps
compile). riak-like projects can extend the list of recursive commands
(to include 'eunit' and 'compile') by adding
{recursive_cmds, [eunit, compile]} to rebar.config.

While at it, update getopt.erl to git rev 78eda3b.

For now, this is just a proof of concept; would make sense to add a lot
of things, such as quick access to invoking rebar itself (with approproate 
code reloading), as well as an eunit-aware version so that tests could be 
run interactively.

version info

To further support OTP releases I have added support for generating
application appup files. These include instructions that systools uses
to generate a relup file which contains the low level instructions
needed to perform a hot code upgrade. My goal with this module is to
produce "good enough" appup files or at least a skeleton to help one get
started with something more complex. If an appup file already exists for
an application this command will not attempt to create a new one.

Usage:
$ rebar generate-appups previous_release=/path/to/old/version

Generally this command will be run just before 'generate-upgrade'.

To support OTP release upgrades I have added support for building
upgrade packages. Support for this is included in the
rebar_upgrade module, specifically generate_upgrade/2. It requires
one variable to be set on the command line 'previous_release' which
is the absolute path or relative path from 'rel/' to the previous
release one is upgrading from. Running an upgrade will create the
needed files, including a relup and result in a tarball containing
the upgrade being written to 'rel/'. When done it cleans up the
temporary files systools created.

Usage:
$ rebar generate-upgrade previous_release=/path/to/old/version

This also includes a dummy application that can be used to test
upgrades as well as an example.

Special thanks to Daniel Reverri, Jesper Louis Andersen and
Richard Jones for comments and patches.

abnfc is an ABNF parser generator.
Options are:
	- doc_root (defaults to "src")
	- out_dir (defaults to "src")
	- source_ext (defaults to ".abnf")
	- module_ext (defaults to "")

Erroneously put rebar_pre_script at the end of the app_dir modules
list in the previous commit.