Feature/os x framework

This pull request adds a Mac OS X framework which builds Expecta and includes the headers.

Additionally, it adds an EXP_FRAMEWORK_HEADERS define to the framework target and uses it to conditionally import the headers via #import <Expecta/…> instead of #import "…"

The motivating case is my unit test framework [Lagrangian](https://github.com/robrix/Lagrangian), which I would like to include Expecta with, but which is itself distributed as a framework.