Remove deprecated providers.

Remove the FlatDictionaryProvider and the TreeDictionaryProvider as
these were deprecated in 0.2.0.
Bump to version 0.3.0.
Merge branch 'master' into 0.3.0
Add uri generation support.
Adding uri to Concepts and Collections.
Always return urn in lowercase.
Providers add uris to concepts and collections.

A provider can controls the uri of a Concept or Collection. For the
MemoryProvider the uri can be set in the data dict. If there is no uri
present, a generator is used. If no generator was specified, a default
urn generator using a 'x-skosprovider' namespace is used.
Add get_by_uri method to providers.
Add get_by_uri to the skosproviders registry.
Updated HISTORY file.
Merge branch 'master' into 0.3.0
Merge branch 'master' into add_uri
Merge branch '0.3.0' into add_uri
The generate method can receive **kwargs.
Merge branch 'master' into 0.3.0
Merge branch '0.3.0' into add_uri
Add some documentation to the uri module.
Expanded docstring for VocabularyProvider.find()
Creating proper introduction.
Update some API docs.