Impressionist proposals

I have cleaned impressionist.gemspec and added some specifications.
Also I have added impressionist/spec dir, so that we can write tests against our app.
I will also be writing specs, as I am cleaning up the gem following the Single Responsibility Principle.

Our tests would run faster on Travis-ci if we could test impressionist in isolation, without loading rails(i.e rails engine etc)

Let me know what you guys think about those changes.