Timeouts, Constants, HeaderGridView, & Self-Search

- Added custom Timeouts for the OkHttp client
- Moved project's constant keys to main PkRSS file
- Removed unused HeaderGridView and use normal GridView instead
- SearchView inside SearchActivity now works. Search-ception!
Request Callbacks, Tags, and Javadoc

- Callbacks are now set per request as opposed to globally
- Each Request (not RequestCreator) is now assigned a unique ID
- Wrote some basic javadocs for RequestCreator API calls