Provide a working default for test snapshot

[dblock]

Currently requires specifying snapshot name, eg. "FBExampleView":

expect(view).to.haveValidSnapshot(@"FBExampleView");

There should be a way to find the full "path" of an specta test at runtime.

[orta]

As mentioned in reply to https://twitter.com/ceterum_censeo/status/423487687396687872

This might get complex with multiple haveValidSnapshots in the it(@"",^{ }) block. As we could get the string path of the test but we'd have to add indexes.

[dblock]

I think this is all about providing a default. Which should work in simple cases.

[dblock]

Closed via #5.