Swift 2.2 and Xcode 7.3

[phatblat]

General changes to get Nimble to build on Xcode 7.3 without warnings (aside from "Embedded dylibs/frameworks only run on iOS 8 or later")

• Replace deprecated __FILE__, __LINE__ and __FUNCTION__ with #file, #line and #function, respectively
• Use new associatedtype keyword
• Fixed a couple OS X tests that were failing due to @3 not coming through as optional

Supersedes #265

⚠️ These changes require Xcode 7.3 and Swift 2.2 or better.