cast --from-wei and --to-wei should assume unit is eth

[danrobinson]

cast --from-wei assumes the user wants the result in wei, which is an unintuitive default target unit (and contradicts the description in the usage message ("convert wei into an ETH amount").

seth (despite saying otherwise in its documentation) defaults to eth as the target unit, which is more intuitive.

> seth --from-wei 10
.000000000000000010
> cast --from-wei 10
10

--to-wei assumes the source unit is wei, which is also unintuitive and contradicts the usage message ("convert an ETH amount into wei"), although it is consistent with how seth --to-wei behaves.

[gakonst]

For context, I thought that it was OK to diverge from seth / default to wei in the PR which introduced this, #261, but seems like it might've been more unintuitive.