Add validation of uri before saving DatasetURI objects

[mortenwh]

The URI generic syntax consists of a hierarchical sequence of five components:

URI = scheme:[//authority]path[?query][#fragment]

DatasetURI.uri is presently not validated. This could be a big problem if a lot of data is wrongly ingested. We should add some validation in the get_or_create method of this model:

• [x] Check that the generic uri syntax is followed

• [x] Check that the uri points to an actual resource

• [ ] Correct any wrong uris already added to the database - perhaps using migrations?

[mortenwh]

The two first tasks are done in #38