update readme to reflect upstream project change

The `mr` utilily is now called myrepos upstream and they have a new
website address. Update the links in the readme, reflect the new name
of the project where appropriate, and fix the copy surrounding its
introdiction. The actual command is still `mr`.
update Arch install guide to use stable packages

Encourage the use of the stable packages per disro norms. The devel
packages can be used but they trigger full git clones in many AUR
helpers and are generally a pain if you don't specifically want them for
something. Mention the devel package but update the URLs to point to
the one that uses the normal packaging system.
refactor install instructions to include myrepos

Since myrepos is so strongly recommended, included it in the distro
specific install instructions. Refactor the Arch Linux instructions
to better reflect most common usage.
add install instructions for osx

There are other ways to do this, but this seems to be the most commonly
used and easiest way to get the job done. Alternate methods could be
added but at least having this one will alert folks that using this on
OSX is an option.
use more portable find syntax

BSD find (e.g. as found on Mac OSX) has more strict rules for parsing
arguments and needs a path. PWD is not assumed as on GNU find.
Merge pull request #108 from alerque/master

assorted readme updates and other fixes