Convert HTTPLogClient to use libevent.

This removes the dependency on libcurl.
Add a wrapper for evhttp_request.

It's main function is to let you use a boost::function easily.
Fix a bug in the ownership of evhttp_request objects.
Add an async log client, and base HTTPLogClient on it.
Rename the "ct" C++ namespace to "cert_trans".
Run clang-format on the C++ code.
Fix an incorrect option on my previous clang-format run.