json-ld for sesame. 

Added JSON-LD for Sesame, by re-adapting the PR by @artemkozlov in  https://github.com/w3c/banana-rdf/pull/61 to the current code. This is not to abandon the use of Semargl as described by issue #67 , which I would like to do later - after I have finished getting rww-play to pass the LDP tests.
I don't think those tests are ok. There are different ways to relativize URIs in the general case.

For example, one may want `relativize` to return the absolute path when the hosts are the same. That'd be totally valid and I see cases where I would prefer this strategy.

I believe it is still ok in the case of the fragment though. (there is no ambiguity)