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
fix name_first/update tx
fix spec for Script.to_name_new
namecoin wallet: don't spend names, name_list, find tx_hash automatically, fixes
namecoin storage: don't index pk_scripts, more efficient storage logic