Install pyexiv2 via Homebrew

[tjhanley]

I followed this
http://www.leancrew.com/all-this/2011/10/homebrew-and-pyexiv2/

[cooljeanius]

It's also actually available from MacPorts, too, as of https://trac.macports.org/changeset/97517

(just mentioning this since the Readme already mentions MacPorts)