Add validate schema utils

[bhirsz]

Moved validate schema to separate class. It's starting point to having all validations done in respective classes.

Also added unit tests - we're kind of testing not only our validate schema method but also jsonschema validate functionality as well (if they broke something, we should be able to catch it).

Partly implements #41

[bhirsz]

self.validate.schema accepts also path to the file - I will describe this in the documentation so the users could replace our embedded schemas with files if they want to (it will be done automatically once #41 is completed but that's in the future).