Update NIF output to NIF2.0

This pull request includes two changes:
* Finishing the context-hash uri method
* Updating the NIF output according to the NIF2.0 spec which is described in this [paper](http://svn.aksw.org/papers/2012/PeoplesWeb/public_preprint.pdf)

Example of the NIF2.0 output:
Input: `The oil giant Petrobras and U.S. oilfield service company Halliburton trade oil.`
Output: 
```
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix itsrdf: <http://www.w3.org/2005/11/its/rdf#> .
@prefix str: <http://nlp2rdf.lod2.eu/schema/string/> .

<http://example.com/offset_0_80> str:isString "The oil giant Petrobras and U.S. oilfield service company Halliburton trade oil." ;
	a str:OffsetBasedString , str:Context .

<http://example.com/offset_4_7> itsrdf:disambigIdentRef <http://dbpedia.org/resource/Oil> ;
	a str:OffsetBasedString , str:String ;
	str:referenceContext <http://example.com/offset_0_80> .

<http://example.com/offset_50_57> itsrdf:disambigIdentRef <http://dbpedia.org/resource/Company> ;
	a str:OffsetBasedString , str:String ;
	str:referenceContext <http://example.com/offset_0_80> .

<http://example.com/offset_76_79> itsrdf:disambigIdentRef <http://dbpedia.org/resource/Oil> ;
	a str:OffsetBasedString , str:String ;
	str:referenceContext <http://example.com/offset_0_80> .
```
/cc @kurzum