Properly handle const value returns (and constness in general).
Keeps errors when trying to bind a const variables directly,
but const returns should be forced to value-types,
since lua has no concept of `const`.
Merge remote-tracking branch 'root/master'

Conflicts:
	sol/userdata.hpp
Slay some extra spaces.
Fix clang builds due to being unable to use shortcut-syntax for overloaded. You must specify the whole signature (how droll).
Fix unused parameter warnings on clang.