Don't consider '0' to be empty TwiML element content #184

PHP's [`empty()`](http://us2.php.net/manual/en/function.empty.php) returns `true` for `'0'`, which is not desired behavior when checking if `$noun` is valid element content. 

This is the minimal change to fix functionality for issue #184, but other edge cases (like calling `say(false)`) should be reconsidered for expected user behavior.

Still needs a bump in version number.