add graph.adjecentNodes and adjacentEdges
add the filter plugin
add the example
add i/o and fix minor stuff
update README
README fixes
Merge remote-tracking branch 'origin/pr/325'
Minor adaptations for pull request #325

Here are the minor adaptations to merge the pull request:
 - Moved methods `adjacentNodes` and `adjacentEdges` from sigma.classes.graph.js into sigma.plugins.filter.js
 - Moved the related unit tests into unit.plugins.filter.js
 - Added sigma.classes.graph.hasMethod to check wether a methodName already exists or not (and its unit tests)
 - Used hasMethod in sigma.plugins.filter.js to check if `adjacentNodes` and `adjacentEdges` already exist or not