Make comma an `infix` operator.
Use the new comma parsing code in `if` conditions.
Update precedence for `expression` calls that cannot include commas.
Break out of comma expression parsing when a reserved word is seen.
An attempt to ignore assigns in conditionals when in parens.
Try directly setting `x.paren` from `nud` of `prefix("(", ...)`.
Update tests for the new comma expression parsing.

- Need `{"expr": true}` to allow comma expression statements.
- Assignments are okay if not the comma's final subexpression.
- Remove errors due to misparsing of comma expressions.
Add test of warnings for assignments in conditionals.

- Only the last expression of a comma expression will trigger the
  warning.
- Parens around the entire comma expression will prevent the warning.
- Parens around the assignment expression will prevent the warning.
Merge branch 'comma-operator-infix' of git://github.com/usrbincc/jshint into usrbincc-comma-operator-infix

Conflicts:
	src/stable/jshint.js
	tests/stable/unit/parser.js