Add ability to get unspent transactions for the given asset

One of the easiest ways to add value in Java driver is to provide a method for querying unspent transactions for a given asset. This is a very common feature for any use case. So, it makes sense to provide this ability in the driver. Implementation can be to either use unspent_outputs api of bigchaindb server or write logic to optimally fetch unspent output in the driver itself. Currently, it makes sense to use api from the server itself as it is easier to implement and straight forward.