Callback Handler Wrapper

Created a new class specialized for callback handling. I want to be able
to run callbacks on specific threads but having to surround it with
if-else statements each time hurts my OCD.

I ended up writing this wrapper class to handle it in one line of code
instead. It's much neater this way.
Put the CallbackHandler to use

Made sure to use this new CallbackHandler class to use. Code is so much
neater this way.

Also, you may use per-request handlers like you already can with
downloaders and parsers. Enjoy!
Woops

Forgot to include the main PkRSS.java file in my last commit.
Per-Request Downloader Bug Fix

Oops, I seemed to have missed this before.

Previously, the load request would ignore the custom per-request
downloader for generating safe URLs. This is fixed now.