updated for APIv3

unfortunately, this version no longer returns the mergable key
v§ API fix for the merge action
Fixes #5: improved determination of account/repo

Uses `git config remote.<remote>.url` to find the fetch URL directly,
strips trailing ".git", then uses a simpler pattern to match 
account/repo.
Ensure origin is a github repo!
Merge pull request #9 from mrclay/patch-2

Fixes #5: improved determination of account/repo
check if repository still exists
Merge pull request #11 from Mic92/fix-deleted-repo

check if repository still exists
added README
escape shell arguments #8 #7
Merge remote-tracking branch 'splitbrain/master'