Git/mercurial commit SHA extractor added.

gitHeaCommitSha task for git sha.

hgHeadCommitSha task for mercurial sha.

hgHeadCommitNum task for mercurial rev number.

Default settings in CommitNumber also rewrites your version from
'-SNAPSHOT' to '-SNAPSHOT-20140130-<sha>'

Format is configurable using versionFormatter sbt setting.