Allow Disco to return basic types

[shochdoerfer]

Due to the changes #40 it is no longer possible to return primitive types. As pointed out by @settermjd it might make sense to return primitives sometimes.

This requires a few changes:

• it is needed to check if type defined in @return is a primitive type. If it is ommit the type check
• Lazy Beans need to check the type and do not use proxymanager for primitives

It make sense to wait for the ProxyManager 2 / PHP 7.x migration (see #46) before tackling this problem as with the built-in return typehint support it might be easier to handle the type detection.