Support for Beta 4

This PR makes SwiftAutoLayout compatible with Xcode 6 DP4 and Swift's new access control functionality.

The major changes:
* Everything is now public. Not sure if it *should* be (especially all the `relate*` methods), but it *has* to for the unit tests to work (excerpt from [the newest release notes](http://adcdownload.apple.com/Developer_Tools/xcode_6_beta_4_o2p8fz/xcode_6_beta_4_release_notes.pdf)):

> A limitation of the access control system is that unit tests cannot interact with the classes and methods in an application unless they are marked public. This is because the unit test target is not part of the application module.

* Apple seems to have finally fixed the computed property segfault bug, so `ALLayoutItem` is a struct again! Everything seemed to work for me so far.

This also resolves #8 (for me, at least). The actual bug is still in the compiler, but the mere act of changing the type of `ALLayoutItem` from an `@objc class` to a `struct` makes all of the symptoms vanish. When you change the type back, the compiler will still crash, but as a structure everything works swimmingly!