[WIP] Parametric polymorphism proxy / Object method extensions

Interfaces are supported now.

https://github.com/Ocramius/ProxyManager/issues/72 is done with this RP (see https://github.com/blanchonvincent/ProxyManager/blob/642f256850b4bc70f8fdcb840366d13c4ba8f110/tests/language-feature-scripts/overloading-object-method-extension.phpt)

TODO : complete unit tests & documentation