fix encoding in version message for ruby 2.0.0
basic support for namecoin network

can sync the chain and answer name_show requests

TODO:
- merged-mining validations
- namecoin validations
- namecoin address version
- create namecoin tx
receive/send NMC, create and spend name_* tx, better indexing, namecoin testnet
storage: don't cache head block by default

only instances that add new blocks should enable this,
otherwise your head won't get updated if blocks are added by someone else.
fix #get_name_hash
more convenient name_* methods