fix problem with cache being totally removed
Renamed flexibleHeight to layoutFrameHeightIsConstrainedByBounds
This makes it clearer what the property does. The layoutFrame is generally constrained by a view's width, this property also constrains the layoutFrame height. This is needed e.g. for DTAttributedLabel or fixed row height DTAttributedTableViewCell
Updated Demo to demonstrate more clearly static row height versus flexible row height
Improved DTAttributedTextCell

- This now has a mode for static height where the bounds of the cell's contentView determine the size of the text
- Default for hasFixedRowHeight is set to `NO` assuming that more people would use the flexible mode
- Removed the accessoryType parameter from the initializer because that was not needed
- Added warning to requiredRowHeightInTableView if you call this on a cell with static height

closes #304