Added "clone repository" feature

Refer to https://github.com/zaggino/brackets-git/pull/164 for updated PR

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This is the first draft of the "clone repository" feature.

It basically adds a "clone" button near the "init" button and prompt user to insert the Git URL.
It checks if the project root is empty and if so, clones the repository there.

Currently it works but I would like to handle Git errors and convert `isProjecRootEmpty` in a deferred function (with "then" and "fail" etc) but I don't know how to do this.