Prototype the use of auto-complete mode

Fairly large change, based on auto-complete-clang-async.
Other features should be unaffected. The completion-at-point
interface has been completely removed as it was inferior.
let-bind ac-sources for intellisense, instead of overwriting them
Merge pull request #13 from chrisbarrett/ac-proto

let-bind ac-sources for intellisense, instead of overwriting them
Make slightly more user-friendly

Dot completion doesn't work though if the user has other
autocompletions running...
Some minor fixups
Add JSON encoding of completion data