libssh2 1.3.0

This release includes the following changes:

 o Added custom callbacks for performing low level socket I/O

This release includes the following bugfixes:

 o sftp_read: advance offset correctly for buffered copies
 o libssh2_sftp_seek64: flush packetlist and buffered data
 o _libssh2_packet_add: adjust window size when truncating
 o sftp_read: a short read is not end of file

This release would not have looked like this without help, code, reports and
advice from friends like these:

 Henrik Nordstrom, liuzl, Guenter Knauf
 
        Thanks! (and sorry if I forgot to mention someone)
