Add support for options

This patch series contains:
- generic handling of RFC 2347 options,
- support for RFC 2348 blksize option,
- support for RFC 2349 timeout option
- partial support for RFC 2349 tsize option

The only missing bit is support for the tsize option in WRQ requests.
Then please name it "blksize". We can deprecate *my* attribute. I'm also not sure why you need "opt". Can't you just store the values in a "private" $self->{opt} hash, but then expose them as $self->blksize(), ->timeout(), ->tsize() in the public API?