add bitbucket support

Tjaart van der Waltrequested to merge
tjaart/git-commit-insert-issue:master into master

This is my first attempt at getting the plugin to work with Bitbucket repos. I am currently working on getting bitbucket.el into MELPA, and you may not want to merge the code until it has been added, but I thought I would put this merge request out there.

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