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)

If MSYS (with bash) is not installed on Windows then do the shell
variable substitution by ourselves. Otherwise just call bash to do the
job.

Currently the --config switch does not work because when loading
a new rebar config the global setting is ignored for all paths.
This patch provides a check when loading new rebar config to see
whether or not the current config path matches the `base_dir` set in
global conf, which produces the expected behaviour.

Added new property to `eunit_opts` option list:

  reset_after_eunit::boolean() - default = true.
      If true, try to "reset" VM state to approximate state prior to
      running the EUnit tests:
          * Stop net_kernel if it was started
          * Stop OTP applications not running before EUnit tests were run
          * Kill processes not running before EUnit tests were run
          * Reset OTP application environment variables

Previous patch erroneously assumed that cover:start() returned
{already_started,Pid} in the cases where cover was already started. We
now turn {error,{already_started,Pid}} into {ok, Pid} and return
{error, Reason} if we encounter an error we do not know about, this
will cause a nice and violent badmatch to stop everything.