Add concept of Trade to the system

[kumlien]

We need to keep track of individual trades for a Depot. This includes

• [x] Create Trade class
• [x] Create class to represent the Oanda Trade json object
• [x] Add support in OandaApi class for Trades
  • [x] Get all open trades for an account
  • [x] Get a trade by id
  • [x] Close a trade by id
• [x] Update the collection with Trades when opening a trade (open trades are stored as sub-documents of the depot)
• [x] Update the collection with Trades after closing a trade. Move trade to own collection.
• [x] Sync Trades when syncing the Depot (only the open trades are available)
• [x] Controller exposing this resource

Requirements what info to save for a trade is specified in these issues:

• Num #27

[kumlien]

Big ticket... Let's close it now.