1.5.1

680ba67 Merge pull request #32 from shenxn/callback
a8c4fe4 Add on_message callbacks
1369be6 Merge pull request #31 from shenxn/multi-type-preference
1706bd1 Add multi-type support to set_preference
d887a81 Fix error "dh key too small" with OpenSSL 1.1.1 (#27)
b013847 Fix send_command arguments (#22)