Fix osx target

This pull request fixes a few things with the OS X framework target as it’s currently not building in Xcode.

- Add a `Masonry-Info.plist` file which is required to create a framework bundle.
  - As I couldn’t find an URI for you, @cloudkite, I used `io.github.cloudkite.Masonry` for the bundle identifier. Let me know if you want some other URI and I’ll change it.
- Set the base SDK to be the latest available (the recommended approach by Apple and also the only way to make it easy to build, i.e. fixing the build on Xdacted which lacks the 10.7 SDK).
- Set the deployment target to 10.7.
- Set an install path for the framework so that it can be loaded by an app.
- Copy the header files to the correct location so that they can be `#import`ed. Header files can and should be bundled with the framework.
- Separate the target name from the product name.
  - This allows us to get the following build output `Masonry.framework` for OS X and `libMasonry.a` for iOS.
- Make sure the build process can find the OS X frameworks (i.e. `Cocoa.framework`) wherever the user has installed `Xcode.app`.

:beers: