Add "head" segment

Hi,

This is the first time I've used github's pull request thingy, hopefully I've got it right.

This adds a segment to display the abbreviated sha1 of the current HEAD. Currently its only implemented for git, I haven't looked at hg support yet and I don't know if it really maps to anything under svn. One thing that I haven't figured out is how to have the segment hide when in a detached HEAD state (currently you get the sha1 twice).