Build feed downloading functionality into feedapi.module

Currently parser add-on modules are required to implement their own feed downloading functionality. This leads to code duplication between parser add-ons. Implementing a feedapi_download() function (using drupal_http_request()) with built-in caching (both file and database based) support may become usefull.

Some feeds may require additional HTTP headers. This can be addressed by a hook_feedapi_header_alter() hook.