Graph Elements no longer have _keys property
Add Gremlin.addLine() method
Action arguments are now passed as an argument to the add, update delete methods of graph elements. ActionHandlers constructors no longer require an array of arguments as last parameters.
Remove handleAction() method
Append dynamically generated Gremlin lines to Gremlin script

- add Gremlin.computeTransactionScript()
- add ActionHandler.stringifyArgument()
Add non-enumerable writable txid property on graph Elements
Rename delete action to remove
Remove updateEdge and updateVertex methods
Remove all ActionHandler and ElementFactory classes. Move stringifyArgument() from ActionHandler class to Transaction class.
Update the API for addVertex() and addEdge. Pass instance of Gremlin to Element constructor
Fix missing foo property test when adding a vertex
Remove updateVertex and updateEdge tests.
Fix adding multiple vertices and edges in a single transaction
Remove TransactionCommitter class. Rename Transaction test files.
Switch calls to commit() to Node's callback style instead of promise