Adds function to set presence explicitly, query presence explicitly and
to set presence explicitly on sync as well as receive presence updates.

Hunter wants you to set the standard via a toolchain file, but some
distros may override that. See #30.

Rename RelatesTo for reply messages

The bug fix for the race condition on handler execution used the new
strand interface. While it is possible to use bind_executor on every
call, that is error prone and not the suggested usage anymore.

memory leaks in the implementation.

I think I finally worked out the issues w/ linking against the
nlohmann JSON library.  I have updated the build and the various
CI scripts.

This should address comments in Nheko-Reborn#10