support GitHub-style namespacing (org/repo) for bower register

[ohcrider]

for #1857

[mohsen1]

It's already possible to install via org/repo syntax. I would assume you can reuse the convert url logic from that instead of writing your own convertGitUrl

[ohcrider]

@mohsen1 thanks your suggestion,I found install via org/repo is only available the repo in github,but repo is not only in github,maybe it is in bitbucket,my solution is fetching the first remote address that the list of git remote -v

[sheerun]

A test, please :)

[sheerun]

Superseded by #2248