added client-server example

This code uses a packet format very similar to DNSCurve, but it could
be used with any number of stateless protocols.
fixed the nonce generator to deal with bad clocks
added password-based key encryption using bcrypt.kdf
added a box for protecting keys with a password

BCryptBox uses bcrypt for a key derivation function
AFBCryptBox expands the keyfile from 128 bytes to 16 kibibytes to
thwart forensic recovery of a deleted keyfile
added public domain dedication to examples
bug fixes and cleanup

added testing code
made AFBCryptBox work
renamed logrounds to workfactor
got rid of debugging print statements