change default network to "Base"

...since it won't be used for the command-line (yet).
add File storage class and tests
make tests work by faking storage
update File to write wallets to different files

the walletId is the filename
make my code work with the latest interface changes

...to Wallet and WalletFactory
Merge pull request #70 from ryanxcharles/feature/command-line-3

Command Line Interface #3