Rebuilt HTTPClient class as an extension of PEAR HTTP_Request2 package, adding redire...

Rebuilt HTTPClient class as an extension of PEAR HTTP_Request2 package, adding redirect handling and convenience functions.
Caching support will be added in future work after unit tests have been added.

* extlib: add PEAR HTTP_Request2 0.4.1 alpha
* extlib: update PEAR Net_URL2 to 0.3.0 beta for HTTP_Request2 compatibility
* moved direct usage of CURL and file_get_contents to HTTPClient class, excluding external-sourced libraries
* adapted GeonamesPlugin for new HTTPResponse interface

Note some plugins haven't been fully tested yet.