The target_dir config in reltool allows you to put your release in
a directory other than in ./NAME, so we should look in the parent
directory of that to find the new and old versions instead of
simply looking in ./

Move untaring and retaring into a temporary path instead of in ./
to prevent name collisions with "releases" and "lib" that might
exist already. Having a subdirectory rel/releases/ can be useful.

Allows customizing the arguments handed over to the Erlang VM, e.g. to
set a name or a cookie, e.g. by setting escript_emu_args to:

%%! -name rebar -setcookie rebar

- Port compiler no longer requires bash on windows.
- It's possible to use compilers whose command lines don't fit into
  the default template

Calling erlang:system_info(wordsize) yields the internal word size of
the Erlang emulator. But due to the halfword emulator, need to pass
{wordsize, external} instead to get the word size, or pointer size, as
seen by external code such as NIFs. The halfword emulator has 4 byte
internal words but 8 byte external words due to 64-bit compilation,
which means NIFs for the halfword emulator also have to be compiled
64-bit. But just passing wordsize is equivalent to passing {wordsize,
internal}, which does not indicate the pointer size for the halfword
emulator.

Older versions of Erlang do not support {wordsize, external}, though,
so continue to pass just wordsize for those versions.

Change the second parameter of a regex tagged port_source from being a
filename or wildcard to being a list of filenames or wildcards.

Previously: {"R14", "c_src/*.c"}
Now: {"R14", ["c_src/*.c"]}

Motivation for change is to avoid repeating regexes.

get_apps/3 now returns which apps have been added, removed and ugpgraded
in a reasonable way. It should prove more usable should we want to
access any of those lists in future appup related changes.

This patch makes a small change in rebar_core that checks the list
of valid plugins to see if any of them export a pre/post processing
function for the current command. This logic is applied only to the
plugins and allows plugin authors to hook into rebar's execution by
using a naming convention that matches the one used for scripting hooks.

Example:

```erlang
-module(my_rebar_plugin).
-export([pre_compile/2]).
pre_compile(Config, AppFile) ->
    rebar_log:log(debug, "PRECOMPILE: ~p:~p~n",
                  [AppFile, Config]),
    ok.
```

The logic changes from strict overrides to a hybrid for merging os_env
and defaults, whereby defaults are chosen if they are not set in the
os_env or if the defaults contains substitutions (at which point os
environment is substituted).

This still means that rebar.config works as it did before, where it
overrides or substitutes based on the use of $VAR, but that default
and os environment merging works such that the common cases of
providing CC or LDFLAGS in the users environment or command line work
as you might expect.  In that CC is overriden by the os environment
unless rebar.config overrides it, and LDFLAGS from the user environment
is appended or prepended based on defaults and rebar.config.

Addresses problems with https://github.com/basho/rebar/pull/71
Also keeps the fix for bug 255.