@dblock => allow specta extras, and a simpler image folder setting

In reference to https://github.com/dblock/ios-snapshot-test-case-expecta/issues/1 I've simplified the API for setting the images directory to a c function that feels a lot like the rest of spectra. So you do `setReferenceImageDir("blah")` like expected, and the docs now state that this only has to be done once.

In reference to https://github.com/dblock/ios-snapshot-test-case-expecta/issues/2 I've extended the API for expecta if we see that you've included Specta in CocoaPods, this is a weak dependency because expecta is built to work without Specta, and so should our library. If you don't have it installed it will always fail.

This means we offer:

```
haveValidSnapshot()
recordSnapshot()

haveValidSnapshotNamed()
recordSnapshotNamed()
```