Teach Foolscap to use twisted endpoints

Hi Warner,

It might be a bit premature of me to send a pull request... I had planned to squash many of the intermediate commits... but can I at least get a code review?

In this branch I have ported Foolscap client side to use endpoints... AND I merged str4d's
server side Foolscap patch. Let me know what you think so I can make the needed changes to get this merged!

Cheers,

David