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.
Can you name it `parse` or `fromString`?