Update NotEmpty validator to use bitmasking

This updates the NotEmpty validator to use bitmasks instead of adding and subtracting the values. While it may not seem like a big deal, there are bugs that can crop up if a user creates code that adds in the same value more than once. For instance, without this patch, setting the type to array('boolean', 'boolean') means that the validator will behave as though you'd set it to array('integer'). 

Additionally, there's no reason to subtract the value of the bit mask from the $type value since we can use bitwise math to determine which flags are set. Using |= instead of adding means that even if we set the same flag lots of times, the value still will be the same. I've added a new unit test which will fail with the current NotEmpty implementation but will work with the updated implementation.

The documentation needs to be updated as well since the NotEmpty docs talk about adding the values together which is not the correct way to use these flags.

The values for the flags have also been updated to hex as it makes it a little more clear what's actually happening. Each validation value is a number represented by a single bit (with the exception of the combined values like PHP and ALL). I did not updated the 493, but it should probably be commented or otherwise indicated the values that are being checked by default (boolean, float, string, empty array, null and space).

The only other test changed was to add 3 more checks to ensure the default setting treat 0.0 as invalid, 1.0 as valid and an empty stdClass object as valid.