traversal(_tree) vertex filtering

added params "filterVertices" and "vertexFilterType" to traversal(_tree).  "filterVertices" accepts an array of examples similar to "followEdges", while "vertexFilterType" defines how to filter out vertices that do not match the examples. "vertexFilterType" can be set to "prune", "exclude", or (default) ["prune","exclude"].
Update ahuacatl.js

fix indentation
Update ahuacatl.js

followVertices changed to filterVertices
Update ahuacatl.js

vertexFilterType -> vertexFilterMethod
Merge branch 'devel' of https://github.com/dajester2013/ArangoDB into dajester2013-devel
fixed reference to unused variable
Merge branch 'devel' of https://github.com/triAGENS/ArangoDB into devel
updated CHANGELOG