Allow null to be passed to nullable options and remove \ prefix from return and argument types

Original report: https://github.com/couchbase/php-couchbase/pull/28

A lot of API calls end with some nullable $options variable. But adding null would give an error.

For example:

Fatal error: Uncaught TypeError: Argument 2 passed to Couchbase\Collection::get() must be an instance of Couchbase\GetOptions, null given in /opt/php-couchbase/tests/d.php:26

I've added a ! to the zend_parse_parameters call to allow null to be given.

I also had to make the arguments nullable in the definition in a few cases.