Add Vertex and Edge classes which extend Element
Add _type to graph Element
Fully refactored cud() with element handlers, updated postData() accordingly
Switch to a higher level http request library (add dependency to request v2.27.0)
Extract TransactionCommitter class from Transaction, added several inner methods
Group transaction files in same folder
Fix some typos
Add postBatch, postVertices and removeVertices methods
Reorder TransactionCommitter methods, add comments, clean up files (entrendipity/grex#19)
Merge branch 'refactor-transaction' into develop