Changed SickbeardAsyncTask to use executors

SickbeardAsyncTasks now use the default thread pool executor by default
and the EpisodeSearchTask uses its own executor with a bigger pool size.
Changed HTTP requests to use a single pooled client
Fixed season container invoking context menu

Fixes #21
Merge pull request #25 from rastating/enhancement/thread_pool_refactoring

Enhancement/thread pool refactoring