Added a types attribute to annotate() and candidates()

Added a types attribute to annotate() and candidates(), which enables server-side filtering of resources. It also makes for a nice addition to the policy parameter.

I've tested it on both kinds of backends, but it only works properly with the Lucene-backed web service. This is, however, not a bug in pyspotlight and seems to be an unnoticed bug in Spotlight's statistical backend. It will be discussed in [Spotlight issue #251](https://github.com/dbpedia-spotlight/dbpedia-spotlight/issues/251).