Put the "Transactions" module behind an option setting

Hi @vandre, I'm a big fan of this. I wanted to allow the transactions module to sit behind an option like the other modules.  

I tried to do quite a bit of testing, but I recommend doing a pass on your own before pushing to the store in case I missed something. 

Notes:
- I separated out the transactions module code into it's own file. 
- I removed some of the extra code copied from a stack overflow answer that wasn't needed.
- ~~I changed $.get into a global httpGet method. While jquery wasn't used in this extension, I made the mistake of assuming [jquery.get](http://api.jquery.com/jquery.get/) when I saw $.get, when $ was just bound to getElementById~~ Switched back per @vandre request.

I look forward to any response.