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
declare json package dependency
fix failing tests
remove cl from completion, fix byte compiler warnings
remove json packages
Merge pull request #14 from chrisbarrett/json-deps

Json deps
Add JSON output selection command to fsautocomplete