Rewrite the Swift class support in Objective-C

Turns out that you can't use Swift in a framework on iOS 7, so move everything
but the convenience functions in Objective-C and switch to building a static
framework. The remaining Swift code is left in a single file which must be
included directly in the user's application (if they want to use it).

This happens to add support for optional RLMObject properties in Swift.
Make the path to Xcode overrideable with the XCODE_PATH env variable
Clean up some stuff
decouple code paths for parsing properties for objc vs swift
Fix running iOS tests from Xcode 6
Update changelog
Merge pull request #901 from realm/tg-swift-ios-7

Rewrite the Swift support to work with iOS 7