Fix Xcode 6 crasher caused by invalid KWExample test invocations

For background, see https://github.com/Quick/Quick/issues/1.

Xcode 6 uses the test invocation's selector, rather than the `-name` property, to display the test in the navigator. When Kiwi creates a test invocation, it is not setting the selector, and ultimately Xcode winds up messaging `NSNull` and crashing.

This PR moves the responsibility for unique selector name creation into `KWExample` and `KWExampleSuite`. It also ensures that examples in the same suite that wind up having the same selector name (most common when using anonymous it nodes) are assigned a unique sequence number to differentiate them.