Adding BitShares support

[Adman]

• What is the approximate scale? We would like to add just getPublicKey and simple transfer operations and maybe extend it to support more operations in the future.

• Do we need any new cryptographic primitives to support this coin? No, we just need to add is_canonical function when signing with secp256k1.

• BitShares docs: http://docs.bitshares.org/

• Pybitshares docs: http://docs.pybitshares.com

• Signing transaction: http://docs.pybitshares.com/en/latest/bitsharesbase.signedtransactions.html#bitsharesbase.signedtransactions.Signed_Transaction.sign

• Explorer: https://bitshares-explorer.io

[prusnak]

We don't want to add coins that do not belong to TOP 30 coins at the moment.