Fee and dust

This PR 1) fixes the constants used for Bitcoin transaction fees & dust and 2) adds fee estimation for dogecoin and litecoin as an example of how it can be done for alternative networks.
Fixed in 5dcefc53291086a839cac8ca7c58eedf2152e085. I removed its dependency on fixtures entirely and used stubs instead. After all, why should fee estimation care about how transaction is serialized? It assumes tx.toBuffer does its job correctly and produces the correct size.