Integration tests using test-kitchen and serverspec

Some pretty basic test cases for both source and mesosphere as well as master and slave recipes, that are currently a bit too coupled to the actual execution of the recipes of this cookbook.

My reasoning behind the tight coupling is to detect breaking changes quickly when adding support for more platforms to this cookbook. Also, it would probably be beneficial to extract some integration tests to unit tests (using ChefSpec) to favour a faster development cycle. Examples would be contents of configuration files.

I added a blurb about testing to the README about the above motivation as well.

Watch out for 934dd9140556f1258077c978aca095d977f1e10b which actually adds a package (`libcurl3`) to `mesosphere` recipe as it was needed by `dpkg` to actually install mesos from the mesosphere package. That commit also changes a line in `slave` recipe to use `Chef::Application.fatal!` instead of `Chef::Log.fatal!` as the newer versions of chef doesn't have a `fatal!` method on `Chef::Log`. I wouldn't have any problem with reverting that particular line change.

Closes #8.