sizeDirty flag removed

https://github.com/Famous/famous/pull/186

https://github.com/contra/famous/blob/0fe835dd391142f188435d41979ae0b2205fe9d7/core/ElementOutput.js#L254

The context's size, referenced as `size`, is very often not equal to `this._size`. When this is the case, `this._sizeDirty` is always true, and leads to exponential sizing. Removing the `_xyNotEquals` check solves the issue. 

@michaelobriena 
@TheAlphaNerd