Moved readme to asciidoc format
fixed block sizes for hmac-sha2-256 and hmac-sha2-512

Both MACs have to use larger block/digest sizes than SHA1.
Additionally the KEX must be changed so that it will resize
the keys "E" and "F" to get keys of the right size for those
MACs (according to section 7.2 of rfc4253)
Add constructor with subsystem name parameter.
Merge remote-tracking branch 'boris/fix_hmac-sha2'