Custom Design File

I needed to update some colors so I made some modifications to change the default design file : 

- The path to the podspec is now relative and not hardcoded with your setup path.

- Following a deprecation warning of cocoapods I edited the Podfile

- The default design path is now a static NSString and is editable with a class setter.
- I had to delete the test target because the files are not on the repo, I couldn't build the Demo project :(