Merge pull request #7 from capitaomorte/master

Also explicitly require 'auto-complete
Provide option to display symbol flags in completion menu

eg. "reduce  -f------"

Closes #8
Fix broken defgroup form
Add screenshot
Make prefix-finding function customizable

* ac-slime.el (ac-source-slime-fuzzy): use
`ac-slime-symbol-start-pos'.
(ac-source-slime-simple): use `ac-slime-symbol-start-pos'.
(ac-slime-symbol-start-pos): new function.
(ac-slime-symbol-start-pos-fn): new variable.