Fix commit author email for bitbucket

Fixes #602.
Email is parsed from raw_author field in bitbucket post-commit hook. It's whatever user has in `git config user.email`. 
I could also check if the value provided is valid email and in case it's not - fallback to whole `raw_author` field or just `author` as it is now. I'm not sure it's necessary, though.

Any comments appreciated.
we can break this out of the function as a package-level variable so that it doesn't recompile the regexp every time. We can also use the `regexp.MustCompile` to avoid the empty `_` for the ignored error