wrap libssh2_knownhost_* functions

Add new Net::SSH2::KnownHosts class.

Wrap functions for reading and writing the known_hosts file, for
checking host keys and for adding them to the known_hosts file.

Add Net::SSH2 method known_hosts which acts as a factory for
Net::SSH2::KnownHosts objects.
rename hostkey method as hostkey_hash and warn when the former is used
wrap libssh2_session_hostkey as remote_hostkey
readfile returns the number of entries read on success
add new set of constants required by the new known_hosts feature
update copyright information