Proposed Datastore API

#90 #119 
This is the DataStore API to be exposed to Query Engine.
DataStore is a singleton that pushes Operations to the deque<Operation> that will later be committed by Transaction.
Transaction ensures that there is only one instance of DataStore that is accessing/modifying the xml tree at any point of time.
Operation classes coming in the next PR
So here, `Transaction t(DataStore::begin())`. Looks weird tho =/