Followers + authentication errors

[kdonovan]

Thanks for the redvine gem! This pull request adds the #followers and #following methods + full specs for them both.

It also raises a custom exception detailing what went wrong when authorization fails.

[kdonovan]

Added passing an option to #connect to skip raising the exception to make existing code easier to port. Also improved the custom exception to extract the code separately from the error message.

[kdonovan]

Also raising a custom exception if the user tries to make API calls requiring authentication before actually authenticating. Without this change, if you forget to call client.connect before any other API calls you end up raising an obscure "can't call strip on nil" error from Ruby's standardlib (tracked down to passing {'vine-session-id' => nil} as a header).

[stakes]

Awesome - thanks for the contributions and improvements! I'll merge this and then bump the version of the gem itself tomorrow.

[stakes]

Here you go sir: http://rubygems.org/gems/redvine updated. Happy to add you as an author as well if you like!

[kdonovan]

Thanks, Jay! I'm happy submitting pull requests for now, but I'll let you know if we end up needing to make any major changes. :)