Add option to avoid shallow git clone

[michaeltyson]

Shallow git cloning was introduced into cocoapods-downloader in August this year, which breaks any HTTP git repository, like ours.

It would be nice to have an option to disable this via the Podspec so we can continue to offer CocoaPods support for HTTP git repositories.

[michaeltyson]

A note: It is possible to use the :commit option instead of :tag or :branch, and this will force a checkout instead of a clone, which works with HTTP git repositories. So this isn't urgent, but would be nice.

[kylef]

@michaeltyson can you elaborate on how it "breaks any HTTP git repository".

[segiddins]

This was fixed by #36.