can now compute balances and save them to db

updated function:
* dbupdatediffsbyblock - now computes balances based on previous balances,
since each address has a "balance" for each block, which is the diff for that
block plus all previous blocks. this calculation works so long as all previous
blocks have been updated.

added functions:
* dbgetdiffbyheight - gets diff (a db object containing the diff value, and
balance, and other stuff)
* dbgetbalance - gets balance (just a value) for an address, at highest
available block

updated example:
* examples/printgenesisdiffs.js

...and added tests for added functions