Allow this framework to be bundled more flexibly on OS X

(e.g. as a subframework within another bundle)

See these for more discussion of why this is better:
http://overooped.com/post/87887322/use-rpath-instead-of-loader-path-or-executable-path
http://www.dribin.org/dave/blog/archives/2009/11/15/rpath/

In my case, I needed SocketRocket to be a ± hidden dependency within a site-specific framework and the app could not be deployed with the previous installation path. (simply using `@loader_path` does work because within an app it would be at `@loader_path/../Frameworks` but within a parent framework it needs `@loader_path/Frameworks`.)