New formula for pixz.

Pixz (pronounced 'pixie') is a parallel, indexing version of XZ.
This formula successfully builds on OS X 10.8.3. It requires libarchive.
Added an option (--build-docs, hat tip to whoever did the git-cola
formula as well as the helpful folks in IRC) which will build and
install a manpage from the asciidoc source the author included.
Unfortunately, building the docs requires the installation of 2 other
packages, so I decided to keep it optional.

Updated formula to meet cookbook guidelines.

Replaced /usr/local with  HOMEBREW_PREFIX, removed extraneous comments
and added => :build for the packages required to (optionally) build the
docs.

Added dependency for xz