Switch from cpp-netlib to libevent.
Point to libevent libraries.
Fix signed/unsigned comparisons.
Use static_cast.
Add FIXME.
Refactor to avoid exposing Base::base_.
Eliminate unneeded get().
Have Monitor keep HTTPLogClient as a pointer.

Also some minor cleanups.
Do a signing run at startup.

This makes sure that we do not serve invalid empty tree heads (fixing
issue #143), as a side-effect.