Move `project_root` and `source_root` up to main module.

Since these values cannot be changed/configured by the end-user it makes
more sense for them to live on the main `Omnibus` module.
Add a `Omnibus.root` -> `Omnibus.source_root` alias

This is mainly there for backward compatibility for older Omnibus
projects that use the older naming.
yarddoc format fixes
Add a `.yardopts` file and ignore generated `doc` dir
Add a Rakefile for bundler, rspec and yard tasks.
Add `Omnibus.omnibus_software_root` helper

This method is meant to act as the 'one true way' to retrieve the
location of the `omnibus-software` gem if it exists in the gem path. We
also find this using `Gem::Specification#find_all_by_name` which should
work in non-Bundler situations.