Various fixes from #1

Changed naming convention from c1 and c2 to lhs and rhs (left hand side,
right hand side).

Fixed incorrect __index assignment

Removed the unused 0 and 255 parameters in the Color() function

Fixed accidentally asserting c2Type (now rhsType) twice in __sub, __mul,
and __div

Removed unnecessary Round and Clamp in __add, __sub, __mul, and __div
(the Color() function handles rounding and clamping automatically).

Removed __gt and __ge (not called by Lua)

Removed commented alpha field code