INSERT containing GRAPH with variable for name

Using ```INSERT { GRAPH <foo> { ... } }``` one can instert data into a specific graph. The name of this graph can also be given by a variable, e.g. ```INSERT { GRAPH ?g { ... } }```. This fails (note that the graph has no name):

```
Python 3.2.3 (default, Jun 25 2012, 23:10:56) 
[GCC 4.7.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import rdflib
>>> import rdflib_sparql.processor
>>> g = rdflib.ConjunctiveGraph()
>>> rdflib_sparql.processor.processUpdate(g, """INSERT { GRAPH ?g { ?g ?g ?g . } } WHERE { BIND(<http://1> AS ?g) }""")
>>> g.serialize(format='trix')
b'<?xml version="1.0" encoding="utf-8"?>\n<TriX\n  xmlns:xml="http://www.w3.org/XML/1998/namespace"\n  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"\n  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"\n  xmlns="http://www.w3.org/2004/03/trix/trix-1/"\n>\n  <graph>\n    <triple>\n      <uri>http://1</uri>\n      <uri>http://1</uri>\n      <uri>http://1</uri>\n    </triple>\n  </graph>\n</TriX>\n'
>>> rdflib_sparql.processor.processUpdate(g, """INSERT { GRAPH ?g { ?g ?g ?g . } } WHERE { ?g ?g ?g . }""")
```

This pullrequest fixes this problem. Note that I have not done any tests. It could be that my fix is incomplete or introduces other problems.