Adding a conceptscheme view.

Adding a view for a conceptscheme. This will be the entrypoint for
building lists of concepts or searching on them.
Adding a concept view.

Adding a view to offer detailed information on a single concept.
Add dependency to skosprovider 0.1.2.

With skosprovider 0.1.2 we can use the get_provider method on the
registry, which makes for clearer code and will simplify error handling.
Update travis-ci config.