Update: Add no-cond-assign option to disallow nested assignments in conditionals (fixes #1444)

For now, the options are named `parens` (only allow assignments in parentheses) or `never` (never allow assignments). I'm happy to change the names.

I tried to clarify the `no-cond-assign` docs in the course of updating them. A few not-so-obvious doc changes:

+ Standardized on "conditional statements," which is consistent with the [ES 5.1 spec](http://www.ecma-international.org/ecma-262/5.1/#sec-12), rather than "conditional expressions."
+ Removed a large section about how Esprima parses `for` statements, because it seemed more distracting than helpful.
+ Removed a bad link to jslinterrors.com.
Fair enough. I changed the name to `ERROR_MESSAGE`.