Add destructor function to user object holders

[billyquith]

Currently UserObjects have to be manually destructed. Since they have shared_ptrs referencing the holders we should be able to automate this.

This could also be used for placement new, where destruct must be called instead of destroy.

Added experimented unique_ptr for UserObjects.

[billyquith]

Lua binder has __gc support now and objects are garbage collected properly.