Only send blksize and tsize if requested.

See rfc2347 (TFTP Option Extension), rfc2348 (TFTP Blocksize Option),
and rfc2349 (TFTP Timeout Interval and Transfer Size Options).

Reflect blksize requested by client.
Merge pull request #9 from mmattioli/master

Fixed a bug with DHCP proxy mode, created separate LICENSE.md, minor improvements to README.md
Removing no longer needed initial for sendblock
Merge pull request #10 from ibobbitt/tftp-option-blksize

Send TFTP options only when requested, respect blksize
remove useless code
fix server.py indentation.
update from original