Enable responses to be fired on the same thread which was used in the request.

This is handy in situations where more work needs to be done on the thread which doesn't require UI changes, like for example processing JSON responses, parsing images, etc.
Add another constructor for controlling usePoolThread.

No need to construct a looper, or get the current looper, when, in essence, we don't wish to use one at all.
Merge pull request #765 from fineswap/UsePoolThread

Enable responses to be fired on the same thread which was used in the request.