Create hex2bin.js

hex2bin() is new in PHP 5.4.0

http://php.net/manual/ro/function.hex2bin.php
Fix a typo

Fixed a small typo: missing "=" sign
Return FALSE if non-hex char detected

Return FALSE if non-hex char detected - same behaviour as PHP version of hex2bin().
Update comments to properly run the test
Merge pull request #194 from duzun/master

Added hex2bin.js