`--testnet` flag

[coolaj86]

I believe that everything is in place for this to work on testnet (all magic bytes are supported through DashKeys.js), but we need to actually implement the flag and check that the examples can run all run on testnet.

Promo Code masternode:

• https://faucet.testnet.networks.dash.org/
• http://faucet.test.dash.crowdnode.io/

Will also need to put a testnet flag on dashwallet-cli (still has hard-coded values for COIN_TYPE).