Atomic: cleanup and add more operations

Cleanup:
	* Remove volatile keyword, it is of no use at all. [1]
	* Remove the enable_if stuff. It had no use either.
	  The most likely explanation why the enable_if stuff was there is that it
	  was used as something like a STATIC_ASSERT to verify that sizeof(T) is not larger
	  than sizeof(void *). This check however is not just misplaced in a place where we
	  already use a lock, it isn't needed at all, as gcc will just generate a call to
	  to the runtime if it compiles for platforms that don't support atomic instructions.
	  The runtime will then most likely use locks.

Code style fixes:
	* Prefix name of the mutex
	* Line everything up nicely, where it makes things look nice
	* Filling \ continuations with spaces is code style rule

Added operations on the atomic var:
	* Compare and swap
	* Swap

The second point of the cleanup also fixes the Android build of the next commit.

[1]: http://stackoverflow.com/q/2484980