MacOS X library-path detection

Add support for installing on MacOS X.  This is complicated a bit because even the newest MacOS doesn't provide a new enough libssl (i.e. only 0.9.8, not 1.0.0).  It's also a bit complicated to compile your own from scratch, because MacOS doesn't take kindly to installing a newer libssl.  Rather than fight with this, it's most common to use MacPorts or HomeBrew to install libssl, with the caveat that you have to know where to look for it.

This patch adds support to look in the normal places for HomeBrew and MacPorts, before defaulting to some system version.  It also updates the README.txt to mention why it doesn't use the built-in OpenSSL and to give a vague hint at how to get one that works.