This patch adds support for customising the way in which rebar generates
version numbers for app.src files using the `{vsn,Spec}` approach.
Whilst the existing `{vsn,ScmName::atom()}` syntax will continue to
work, users can also pass `{vsn,{cmd,Cmd::string()}}` in which
case the provided *command* will be used. For example:

```erlang
{application, doodah,
 [
  {vsn, {cmd, "git rev-parse --short HEAD"}}]}.
```

This patch corrects the vm.args behavior while building upgrade tarballs
by copying the file from the release into the upgrade. Additionally it
patches the dummy runner script in the upgrade test project to work
properly.

Avoid overwriting custom config (passed with -C) when we are processing
the base_dir in rebar_core.

@ characters causes windows start_erl error

fix vm.args checks by always using $VMARGS_PATH

Get rid of app.config

app.config has been a long standing erroneous file in rebar. Erlang/OTP
documentation suggests a sys.config file instead. This file is stored in
the releases/VSN directory. This does a few things but most importantly
it ensures your config (contained in the application environment)
survives a hot upgrade. It also has the advantage of allowing the
configuration of the application to be versioned along side the
application code. This patch flips rebar to use sys.config rather than
app.config.

Additionally it makes this flip to vm.args as well, making them
versioned just like sys.config.

This patch also includes runner script changes to support the old
etc/app.config config file location and support for Windows.

Thanks to mokele for the initial work and kick in the pants to make this
finially happen.

This patch fixes the warning logging when the number of missing plugins
is greater than one. The current code only works by accident, when a
single plugin is all that is missing.

Export two extra environment variables when executing shell commands.
These variables are useful for rebar hooks that rely on Erlang
applications installed as rebar dependencies.

$REBAR_DEPS_DIR contains a fully-qualified name of the directory where
rebar stores dependencies.

$ERL_LIBS is set to $REBAR_DEPS_DIR or to "$REBAR_DEPS_DIR:$ERL_LIBS",
if $ERL_LIBS was defined before.

This patch updates rebar_core to look for missing plugins (i.e. those
that aren't found on the code path at runtime) in a configurable
plugin directory, and dynamically compile and load them at runtime.

By default, the directory "plugins" is searched, although this can be
overriden by setting the plugin_dir in your rebar.config.

Allows using tools like git-subtree and still maintaining proper
git-based vsns for those "imported" subtrees. Also helps understanding
what was the last change introduced to a specific application within a
multi-application repository (at this moment, all applications that
reside in the same repository, will share the same autogenerated git
vsn)