Add support for SCrypt-based blocks and PoS blocks/transactions

[doc-hex]

I want to expand support in blocks.py for non-bitcoin blocks and transactions, specifically:

• Litecoin (et al) blocks have a hash calculated with scrypt not SHA256
• Blackcoin blocks (and likely many other proof-of-stake altcoins) have a few extra fields in the block header, and transaction serializations.

To do this, I need to add a dependency: ltc_scrypt. See github here and pypi

It's a C module, but it's small, focused and doesn't pull in lots of dependences (ie. it doesn't need OpenSSL).

I could simply add it to pip-req.txt or I could leave it out, and print an error on ImportError when it's needed. Not clear to me which approach is better in this case.

[richardkiss]

Huge delay, but I've done some work to incorporate this. I'm a little stuck on what to do about the dependency, since I've always hoped pycoin to not have any external dependencies. Take a look at the preliminary work here https://github.com/richardkiss/pycoin/tree/feature/blackcoin/pycoin/coins