Bufferevent support for openssl.

Bufferevent support for openssl.

This code adds a new Bufferevent type that is only compiled when the
openssl library is present.  It supports using an SSL object and an
event alert mechanism, which can either be an fd or an underlying
bufferevent.

There is still more work to do: the unit tests are incomplete, and we
need to support flush and shutdown much better.  Sometimes events are
generated needlessly: this will hose performance.

There's a new encrypting proxy in sample/le-proxy.c.

This code has only been tested on OSX, and nowhere else.

svn:r1382