Add support for rpcauth

[sondreb]

See some details here: https://en.bitcoin.it/wiki/Running_Bitcoin

-rpcauth=	Username and hashed password for JSON-RPC connections. The field comes in the format: <USERNAME>:<SALT>$<HASH>. A canonical python script is included in share/rpcuser. The client then connects normally using the rpcuser=<USERNAME>/rpcpassword=<PASSWORD> pair of arguments. This option can be specified multiple times

Code to generate the hash to put in conf file:

https://github.com/bitcoin/bitcoin/tree/master/share/rpcauth

[sondreb]

The Bitcoin Core implementation is located here: https://github.com/bitcoin/bitcoin/blob/d0f81a96d9c158a9226dc946bdd61d48c4d42959/src/httprpc.cpp