Minor fixes to JSON handling

 * Using the native `json_last_error_msg` function
 * Checking tha UTF8 constant exists before using it for PHP 5.2
compat.
 * Renaming exception toggle to `throwJsonExceptions`

closes #44
Few lingering JSON items
Merge branch 'master' of https://github.com/titusfoo/pest

Conflicts:
	PestJSON.php