Use a better way to check for right to left layout direction.

This method to check for RTL layout works with right to left pseudolanguage. Closes #22
Merge pull request #27 from vytis/right-to-left-layout

Use a better way to check for right to left layout direction.
Leverage new APIs to efficiently activate/deactivate arrays of constraints

- Use the class methods on NSLayoutConstraint to activate and
deactivate arrays of constraints. These can be more efficient than
activating/deactivating constraints individually.
- Fix the preprocessor conditionals so that OS X 10.10 and higher can
use the new constraint activation/deactivation APIs when available
Update podspec