Add support for per-language ac-sources

Here are the additions for context-aware auto-complete, per discussion in issue #116.

I did a quick test of web-mode with these changes loaded in an emacs -Q session, and everything else I tried seems to work fine when auto-complete is not available.

Give it a test and let me know if you find any issues.

I can write some docs for setting up ac-sources for web-mode.org, if you'd rather not do it yourself. Just let me know if you'd like it.