Add auto-complete source to Rtags

In response to the dialogue in #162, I've created a working ac-source for rtags completion.

`(require 'rtags-ac)`
`(setq ac-sources '(ac-source-rtags))`

Caveats:
Not yet totally "async" as in clang-complete-async, but performance has been surprisingly acceptable.

auto-complete deletes duplicates when generating candidates, so overloaded constructors or functions were getting cut.  I workaround this by commenting out `(delete-dups candidates)` in `ac-candidates` for the time being.  This keeps the rtags-ac candidates and insertion logic simple.