Fixes #3 - url doesn't need to contain the .git extension.

The implementation will now work with urls like

https://github.com/temochka/sublime-text-2-github-tools.git and
https://github.com/temochka/sublime-text-2-github-tools

The latter seems to be more like a hidden feature, one that some
people are using all the time (including myself).
Improved verboseness of error messages; added handling of http:// protocol

Previously, if a repository couldn't be determined, the user wouldn't
know why that happened. Exceptions now carry more information, which is
presented to the user as well.

Even though uncommon, there might be older repositories out there which
still use http:// , which is now a supported protocol.
Merge pull request #14 from Byron/master

Fixes #3 - url doesn't need to contain the .git extension.