`set_function`/`set_usertype` now properly use `set`
For the time being, we are going to avoid implementing `traverse`; aside from performance of nested table access (e.g. `int x = lua["a"]["b"]["c"]`), it's a pain. ;~;
additional tests to make sure pass-by-value and copy semantics work as intended
new proxy_base class to reduce code duplication
update function / protected_function usage (to solve starwing's issue while keeping code as clean as possible)
sol::object had a few reference leaks in the way it retrieved values: it now does it properly without leaving the stack at +1 item
sol::stack was drastically cleaned up, with the following key change:
* sol::stack::push now returns an integer of the number of things its pushed (usually 1, but can be more) (Thanks, @PrincessNyanara!)
* sol::stack::call now calls functions flexibly, and getting is done more reliably
* due to the innovation of stack::call and using absolute indices, we no longer have to use reverse_call style programming to deal with lua
* sol::reference::get_type is now const-correct
* sol::state and sol::table now have a cleaned up `get` implementation since it is no longer held back by the ugliness of VC++'s incapability to handle templates
* the name `sol::userdata` now belongs to a type that actually encapsualtes a void* with a pusher/getter than gets a userdata void* value (TODO: give it a template to make it static_cast to that type on get?)
* lightuserdata_t -> light_userdata, upvalue_t -> upvalue as type names (mostly details)
* pushers for various types were updated to return integers
It's going to take serious work to make it happen in GCC, plus the fact that 4.9 is still giving me
so many errors I can't even read it...
I'll let Rapptz figure it out, but later.
Sexy class bindings, yes!
