Reimplement click-to-pay. Support OSX.

Switch to using Qt's QLocalServer/QLocalSocket to handle bitcoin payment links (bitcoin:... URIs)

Reason for switch: the boost::interprocess mechanism seemed flaky, and doesn't mesh as well with "The Qt Way".  The new code is shorter and, I think, clearer.
    
I made a couple of extraneous changes (alphabetized some long lists of #includes to make it easier for me to see what new includes I needed to add).

I also added OSX support.

Submitting now for early review, should not be pulled until after 0.8.

Test plan:

On all supported platforms:
  Click on a link when Bitcoin-Qt is not running.
  Click on a link when Bitcoin-Qt is already running.
  Un-gracefully shutdown (kill) Bitcoin-Qt (so local socket file is left behind), then re-run.