Return image from disk if key not present in memory cache. Closes #6

Can someone check the tests? I think they are not working the way they should for me
Can be combined in a single line with `if let`:

```
if let imageData = NSData.dataWithContentsOfFile(imagePath, options: .DataReadingMappedIfSafe, error: &readError) {`
return imageData
} else {
// Log
return nil
}
```