Document skip_apps= and apps=
erlc: fixup log message

Using the filename as a prefix is less readable and inconsistent with
the other log messages.

Before:

DEBUG: src/foo.erl depends on...

After:

DEBUG: Dependencies of src/foo.erl ...
erlc: add test suite
Document require_*_vsn options
rebar.app: remove superfluous quoting
Update rebar_utils.erl

On windows, bootstrap.bat failed with next error.
Command 'escriptize' not understood or not applicable

This happens because the drive name in path got from rebar_utils:get_cwd() and base_dir(Config) are different case.
Made the drive name the same lowercase using filename:absname().
Do not wrap base_dir with filename:absname() 

base_dir() returns already filename:absname()'ed path.
Make sure that eunit/qc_compile_opts works fix #245
Roll rebar version 2.3.0
Merge pull request #242 from tuncer/erlc-speedup-v5-fixup

Extra commits for #129
Merge pull request #274 from redpine50/broken_on_win

Use lowercase for Windows drive name to resolve issue #250
Merge pull request #255 from tuncer/superfluous-quoting

rebar.app: remove superfluous quoting
Merge pull request #244 from tuncer/missing-help

Document skip_apps=, apps=, and require_*_vsn
Merge pull request #251 from mururu/eunit_compile_opts

Make sure that eunit/qc_compile_opts works