improve code from sdk examples -- document / validate hex values, explore sdk for other alt interface

[wildmolasses]

For all these input args where the estimate is supposed to be a hex string specifically, we should either validate that (or does railgun do that?) and/or add comments and/or a more clear variable name. This feels like something that will be confusing to people new to the codebase.

Originally posted by @mds1 in #51 (comment)