Fixes #9, refactor installation

This commit refactors the installation, where it is chosen by an
attribute, with sane default of "package."

This aims to retain backwards compatibility, but due to the age of
previous supported platform versions, we can't assume that it will
work.

* Update test-kitchen w/ additional platforms
* Assures CentOS support
* Split installation methods up into separate recipes
* Add an installation method attribute
Merge pull request #11 from jtimberman/install-method-refactor

Fixes #9, refactor installation