Calabash as dylib

See https://github.com/calabash/calabash-ios-server/pull/52

This pull request adds the dylibs to the Calabash gem and provides `require 'calabash/dylibs'` as an easy way to access those paths.
@krukow 

I am not sure I agree with adding this 'CALABASH_BUILD_DYLIBS' variable.

Either we are going to distribute the dylibs or we are not.

We'd have to make a condition in the .gemspec as well or building the gem fail.