variadic_args, readonly, and property !

This release adds many additional features both imagined and requested, as well as a handful of fixes including:

sol::variadic_args - take a variable number of parameters at the end of a function you bind using sol.
sol::property( &my_class::getter_func, &my_class::setter_func ) - use member functions to implement read-only / write-only (or read-write) variables
sol::readonly( &my_class::some_variable ) - makes a member variable read-only when interacting with set_function on tables and usertype.
sol::lib::ffi and sol::lib::jit - two libraries made available by luajit that weren't included in the standard sol::lib enumeration.

Have fun!