Discuss overloading transaction function calls with gas estimation in contract wrapper

[spalladino]

We currently have a custom sendTransaction method (implemented in #211) that estimates the gas of a tx. We could overload all the functions in a contract that correspond to a transaction to actually use them, so any user requesting a contract from zos-lib Contracts helper has this feature enabled automatically.

This would also allow us to roll back to using the default function syntax, instead of using the sendTransaction raw call.

[facuspagnuolo]

Another alternative could be to provide different instances of the Contracts object and in that case, we will be letting users decide which one they would prefer to use. And it will allow us to use the already known function syntax too.