improvement: transaction building -> gas estimation

[denverbaumgartner]

today: we either utilize the gas value provided by the user or default to a value of 300000 when signing transactions.

soon: we should be dynamically setting gas based on estimation. web3.py has a function to deal with this, but upon trying to implement this function i received values that were significantly underestimated. worth digging back into to see if it was, most likely, an issue in my implementation.