Expand netcode to 3-letter codes, add Blackcoin, related changes

As discussed in issue #29:
- network codes expanded to 3/4 letters.
-`network.py` heavily reworked
- added values for "Blackcoin" as "BLK" although they use "BC" on the exchanges.
- `ku` script updated to handle new feature
- added Coinkite to users list (old news, used pycoin from day 1)
- import bug in testing code fixed