#2 Get Current Balance

[felipeanchieta]

Create a HTTP endpoint which returns the current balance of a given account.

This balance is the sum of all operations until today, so the customer can know how much money they still have.

From the example in #1, the customer would have
1000.00 - 3.34 - 45.23 - 180.00 = 771.43

Acceptance criteria

• Code meeting the requirements and in accordance to Clojure community guidelines as described at https://github.com/bbatsov/clojure-style-guide
• Unit tests with 99%~100% of coverage