README: document dependencies
Move message-encode function to bitcoin/messages module
Store raw blocks as network "block" messages, including msg hdr & checksum
Add mkbootstrap.py script, for creating bootstrap.dat files.
Prefer the function-style print statement
ChainDb: move code to new method .txout_spent
Update BUGS, TODO for recently completed items
ChainDb: replace broken tx_connected logic with orphan detection
ChainDb: check mempool for orphans
TODO: add note on missing opcode
Update for latest upstream checkpoints.
Break out bitcoin library into separate python module.
node: do not send mempool at startup

At least until orphan processing works
New script q_avg_size, example query
q_avg_size: remove unused bootstrap.dat file creation
mkbootstrap: add command line option, to output all blocks,
not just blocks up to latest checkpoint.