Default the network, if not specified, to `networks.defaultNetwork`. Closes #559

- Changes Address and PrivateKey to use the configurable `networks.defaultNetwork` if not specified
- Adds tests to check that the default is being used
- For consistency, networks are name by 'livenet' and 'testnet' in all cases
- Builds browser tests before running to ensure tests are up-to-date when developing
please make this `if (network && !Network.get(network))`, as that is warranted to work whatever it is that the user will send