Simplify encoding/decoding by using Gob

At least for now, using Gob simplifies a lot of code and gets
us where we need to get much faster.
Test key storage
Block database storage
Show last block in the Info report
When mining, use the last block
Block introspection API
Add a primitive test for calculating the merkle root hash in blocks
Block should show Bits as hexadecimal
Minor code improvement