Network: Replaced all instances of mainnet with livenet for consistency
Tests: Enable tests for network, include defaultNetwork, check each constant, build browser tests before running to make sure it's up-to-date
Address: Use defaultNetwork if network is not specified
PrivateKey: Use defaultNetwork if not specified
Tests: Check that an Address is using the defaultNetwork
Tests: Check that PrivateKey is using the defaultNetwork
Merge pull request #644 from braydonf/ref/default-network

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