implement git wip delete

[bartman]

Should be able to delete wip branches.

$ git wip list
wip/foo
wip/bar

$ git wip delete wip/foo
$ git wip delete bar

[snaewe]

What's the 'manual' way of getting rid of any 'wip' branch created with 'git wip'

[snaewe]

I finally found it out: git push . :wip/BRANCHNAME does the job.