Allow expressions with a comma in switch case condition

The ES5 grammar specifies that the `case` keyword should be followed by an
expression (http://www.ecma-international.org/ecma-262/5.1/#sec-A.4) and that
an expression can consist of a comma-separated list of sub-expressions
(http://www.ecma-international.org/ecma-262/5.1/#sec-A.3), so the JSHint parser
should accomodate these commas.
Merge pull request #1586 from jonahkagan/expression-with-comma-in-case

Allow expressions with a comma in switch case condition