Feat/transactions

[tacofordog]

• Created some mock transactions
• Changed Amount field type from Long to Double
• Tweaked display to more closely match Mint's layout (e.g. Amount is located at the end, ID and Type aren't shown to the user, all Transactions initially sorted by descending date order, etc.)
• Moved all business logic to a TransactionService class and altered TransactionResource (aka the controller) to just call the service
• Created most of the necessary filter/sort methods... but not sure how to test / implement them quite yet