Updated gexf-parser, adapted the wrapper

GEXF files are now asynchronously loaded, and the wrapper sigma.parser.gexf.js has been updated.

Related latest version of gexf-parser:
https://github.com/Yomguithereal/gexf-parser/commit/49ab50ebc1d5ff80b5ee69ac5b1a6b9a602328e8
Merge branch 'draft-v1.0.0' of github.com:jacomyal/sigma.js into draft-v1.0.0