Safety fixes and refactoring

[guidanoli]

• Adds LuaRuntime.stack for context managing the Lua stack, which makes the code cleaner and safer
• Calls check_lua_stack where needed (avoids Lua stack overflow)
• Optimize py_to_lua, handling True and False separately because <bint> cast calls Py_IsTrue unnecessarily.
• Partial rewrite of _LuaObject.__str__, _LuaTable._setitem, _LuaFunction.coroutine
• Removed lua_settop(L, 0) from _LuaObject.__call__.
• Renamed push_lua_arguments to py_tuple_to_lua and made it safe against py_to_lua exceptions
• Renamed unpack_lua_results to py_tuple_from_lua and added nargs parameter instead of getting from Lua stack top

[guidanoli]

See #187 for updated changes