Fix license typo in README
Warn when the website is in quirks mode

Summary:
KaTeX doesn't work correctly in quirks mode. Warn in the console and disable
rendering if that happens.

Test Plan:
 - Make sure the test still loads and renders math
 - Make sure a warning is thrown when the doctype is removed, and no more math
   is rendered
 - Make sure the tests pass both on the web and with `make test`

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13192
Add CLA information in CONTRIBUTING.md

Auditors: alpert
Merge pull request #44 from grizzilus/master

Fix license typo in README
Added various set theory and logic symbols

\subset, \supset, \subseteq, \supseteq, \cap, \cup, \setminus,
\neg, \lnot, \top, \bot, \emptyset, \varnothing, \land, \lor,
\wedge, \vee, \notin, \nsubseteq, \nsupseteq, \models