Wrap libssh2_knownhost_* methods

This pull request does several things:

* wraps the basic method for manipulation of known_host files (readfile, writefile, check key and add key) and makes them available through the new class Net::SSH2::KnownHosts. Object of this class are created using `$ssh2->known_hosts`

* rename Net::SSH2 hostkey method to hostkey_hash (it wraps libssh2_hostkey_hash, so this is the right name)

* emits an obsolete warning when hostkey is called

* wraps libssh2_session_hostkey as Net::SSH2::remote_hostkey.

* regenerates const-c.inc using a more recent version (1.4.3) of libssh2 headers so that the constants used with the methods in the KnownHosts class are available.