ENH : add ability to specify git version by sha1

 - allow 'git_sha1' as a valid key in 'source'
 - also grab the full HEAD sha1 and shove it into the environmental
   variable GIT_FULL_HASH
API : replace git_tag/git_branch with git_rev

As @asmeurer these are redundant with each other.

Added a clean-up function which will remove the old tags, and
if they exist will copy the value over to git_rev.

Cleaning function will also raise an exception if more than
one of the tags is given.
Merge pull request #214 from tacaswell/git_sha

ENH : add ability to specify git version by sha1