Fixing GEXF parsing in IE.

IE apparently doesn't support getElementsByTagNameNS for XML documents.
add parseJSON plugin
Merge remote-tracking branch 'fix-ie-gexf/fix-ie-gexf' into devel
Merge remote-tracking branch 'parse-json/parseJSON' into devel
add interpreter command
make sure attr is in nodesAttributesDict
edge ids can be duplicate in gexf exports of directed graphs, thus do not use the gexf edge id but increment a counter same as in parseGexf plugin
Merge pull request #7 from yaph/devel

ie parseGexf fix, parseJson use unique edgeId, python fix for attributes not in dict. Thanks, yaph.
drop id, label from edges; check for edge attributes not in dict; store weight to size parameter
Fix adding of edges to graph in pushGraph (should be addEdge instead of addNode)
Use required ID attribute when parsing GEXF
Merge remote-tracking branch 'origin/pr/85'

* origin/pr/85:
  Use required ID attribute when parsing GEXF
Merge branch 'pr/73'

* pr/73:
  drop id, label from edges; check for edge attributes not in dict; store weight to size parameter
  add interpreter command make sure attr is in nodesAttributesDict edge ids can be duplicate in gexf exports of directed graphs, thus do not use the gexf edge id but increment a counter same as in parseGexf plugin
  add parseJSON plugin
  if edge weight is set, set size attribute to this value to make setting minEdgeSize and maxEdgeSize effective
  Fixing GEXF parsing in IE.
IE doesn't support overrideMimeType
IE doesn't support namespaces and getElementsByTagNameNS for XML, but it is still possible to retrieve namespaced nodes using getElementsByTagName with a 'theNamespace:theNodeName' argument.
FireFox needs a URI, but all browsers support the viz:* syntax as it turns out, so use that
Merge branch 'pr/89'

* pr/89:
  IE doesn't support overrideMimeType
minor change
Added edge arrow rendering
Merging pull request #101: Edge arrow rendering

Added a graph parameter "arrowRatio" that determines the ratio of the size of the arrow head compared to the edge size, and fixed style to make it fit with Closure Linter rules.
added sideMargin in graphProperties
restored sideMargin default value to 0