Mark cluckin 'PDF' option as default.

NOTE:
* It should work without requiring this since we are picking up
cluckin method from socket but it doesn't seem to work correctly first
time.
* We don't want to spend too much time debugging chrome extensions.
Bypass cluckin if keywords (not URL) is keyed into url input bar.

* Currently we bypass cluckin if it is off
* We also want to bypass cluckin if someone keys in keywords
  into Chrome's url input bar because that should go to Google search,
  which we don't want cluckin to process.
Merge pull request #10 from nethsix/bypass_cluckin_for_keyword_search

Bypass cluckin for keyword search