BREAKING CHANGE: API update

GexfParser.fetch(url, ?callback) now fetches and parses a file which URL is specified. It works exactly as the former GexfParser.parse:
  - GexfParser.fetch(url, callback) will load and parse the file at the URL asynchronously, and execute the callback with the graph as only argument.
  - GexfParser.fetch(url) will load and parse the file synchronously and return the graph.
GexfParser.parse(xmlString) now parses a string GEXF file content and return the graph.
Also updated unit tests and added some for the new GexfParser.parse.