Unit/integration testing

I wrote a few more unit tests (actually more integration tests than unit), very simple, to check that the library actually does... well, something.

It has raised a problem: while the tests pass on Windows (see here : https://ci.appveyor.com/project/cosmo0/imageprocessor), on Mono the animated gifs can't be processed. I have fixed a problem but I still have another : http://stackoverflow.com/questions/24470312/cant-create-modify-gifs-with-mono

I should write more precise and useful tests, but at least it's a start :)