Fixed flycheck workaround when completion is aborted(#300)

Thank you for issue reopening. I fixed behavior when completion is aborted.

Fixed points:
- I made to start flycheck's syntax checking when completion is aborted too.
  - I add timer to detect completion is aborted.
  - I add user customizable variable to custom above timer's polling interval.
- I made to load Flymake and Flycheck (if exist) before `defadvice`.
- I splited function `ac-setup` for `ac-syntax-checker-workaround` because it becomes long.