Make feedbag threadsafe.

Unsure why, but it looks like the original feedbag was using global vars for `$feeds` and `$base_uri`, rendering the whole gem unsafe if multiple threads are using it concurrently.  This patch preserves the API, but:
* Promotes Feedbag module to a class
* Moves the bulk of its methods into instance methods
* Removes global vars and replaces them with instance vars.