pass mining fee to transaction builder

[kp666]

add spec for mining fee
refactor code

[oleganza]

I liked the previous API better: all parameters are set during init and build only wires all of them together. Why did you decide to put mining_fee argument in build?

While we are at it, lets rename it to fee, to be consistent with fee_rate.

[kp666]

Well, my thinking was to create a builder object and then reuse this builder object, to build transactions with a different mining fee.