Allow custom view height and properly calculate height in sizeToFit

* changed types of `minHeight` and `maxHeight` to `CGFloat` to maintain type compatibility with `CGSize`
* allow setting `minHeight` and `maxHeight` to custom values
* `sizeToFit` now readjusts frame height if necessary
* fixed issue where delegate method `growingTextView:willChangeHeight:` gets called on text change, even if height did not change.