Resolve some internal implementation issues

- Make the constraint identifier APIs available on Mac OS X, as long as the SDK is at least 10.10 (will work with deployment target of 10.9 or higher)

Fixes #18
- Fix issues with nesting constraint blocks (nested calls to autoSetPriority:, autoSetIdentifier:, etc)
- Don't set the priority of constraints when installing them unless inside a constraints block
Update podspec
Update version number to v2.0.1
Update availability of constraint identifier methods
Update README
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
Add newline to end of file
Add .DS_Store to gitignore
Add support for usage in App Extensions

When using PureLayout in an App Extension, define the preprocessor macro PURELAYOUT_APP_EXTENSIONS in the Build Settings of your App Extension's target to prevent usage of unavailable APIs.

Fixes #30
Bump version number and update podspec
Remove some unused imports
Update travis.yml
Return array of created constraints from -autoCreateConstraintsWithoutInstalling:

Enhance the -autoCreateConstraintsWithoutInstalling: API on UIView/NSView to return an array of all the constraints that were created (but not installed) from calls to the PureLayout API inside of the block.
This is particularly useful since any constraints in the block must be captured and stored somewhere, otherwise they will be immediately released (since they are not retained anywhere else).
Add new demo for creating constraints without installing
Update README
Add API to set the views in an array to a size
Improve indentation for preprocessor directives