Now it retrieves the address balance from the database
instead of the xsn server, this reduces the data because
now we get the balance only and the transactions are retrieved
with another endpoint.
This is a piece for #18, it allow us to retrieve the transactions
for the given address.
NOTE: This commit doesn't include tests in order to work in the frontend
concurrently, tests will be included before the release.
Alexis Hernandez