use `estimateGas` when checking account balance for transaction

[nugaon]

Currently, before sending transaction in registerUsername, it only checks a whether the balance of the address is higher than a constant value 10000000000000000 wei.
https://github.com/fairDataSociety/fdp-contracts/blob/master/js-library/src/services/ens.ts#L114 Instead of that, some other dynamic solution could be used such as estimateGas on the register method.

[tomicvladan]

Implemented in https://github.com/fairDataSociety/fdp-contracts/pull/105