Add support for Swift Package Manager

[natestedman]

This adds support for the Swift Package Manager. Due to swiftlang/swift-package-manager#16, I had to rename the ResultTests directory to Tests. Building and testing in Xcode and with carthage still works correctly.

You can test it with this Package.swift file:

import PackageDescription

let package = Package(
    name: "Testing",
    dependencies: [
        .Package(url: "https://github.com/natestedman/Result.git", majorVersion: 1, minor: 1)
    ]
)

$ /Library/Developer/Toolchains/swift-2.2-SNAPSHOT-2015-12-01-a.xctoolchain/usr/bin/swift build
Compiling Swift Module 'Result' (2 sources)
Linking Library:  .build/debug/Result.a

I had to tag an additional release on my fork to try it out, as I don't believe that the Swift Package Manager currently supports arbitrary branches or commit SHAs like carthage does.

[NachoSoto]

😍😍😍😍

[robrix]

We can rename the group too if it makes things easier. I’ve no preference.

[robrix]

I’m a little surprised we don’t have to do the same thing with the sources as well.

[robrix]

Thank you for the PR!

[swizzlr]

Could this get merged?

[robrix]

Needs a merge from master, to begin with.

[natestedman]

Rebased.

[robrix]

Thanks!