Nuke trailing white space
Git: Added ref option.

The ref option allows you to use a specific revision (usually a tag)
instead of the HEAD.
Git: renamed 'ref' option to 'rev'.
Git: accept revision instead rev as argument.

'rev' is the canonical one.  We complain if both are set.
If 'revision' is set we delete that and add its value in the 'rev' option.
mercurial: add support for branch specifier
Updated docs to include hg branch option
mercurial: do --update instead of --branch
Added suport for the Windows Msysgit.

The Msysgit creates a git.cmd executable instead of a git.exe, therefore the current script was unable to find the Git in the PATH. I solved the problem by addind support for multiple Git binaries names.
Update HISTORY
Merge pull request #53 from canassa/master

Add support for the Windows Msysgit
Fixed git part of readme to talk about the 'rev' option instead of the former 'ref'.
Merge pull request #51 from posborne/master

Support for mercurial 'branch' option
Updated history.
Renamed README.txt to README.rst for nicer github page.
Update image url.
Merge remote-tracking branch 'mauritsvanrees/master'

Conflicts:
	docs/HISTORY.txt
	src/mr/developer/git.py
Pyflakes and whitespace.
Prepare mr.developer 1.18.
Bump.
Do real checks of modules instead of version check.

On many installations python in some version (eg; 2.7) can be installed without
required module (eg. argparse). So instead of relying on version check, do real
requirement check and then optionally add missing requirement.