Discover return types from DocBlocks

[asgrim]

Using the DocBlocks, we can at least attempt to resolve the return types for functions and methods.

/**
 * @return int|null
 */
public function aMethodReturningInt() { /* ... */ }

Implement a new method getDocBlockReturnTypes() which would return an array of Type objects. In this case, it would return something like a [Types\Integer,Types\Null_]

[Ocramius]

Handled in #104