Add an `Omnibus::Util` module for utility methods

First methods up are `shellout` and `shellout!`, nice wrappers around
`Mixlib::ShellOut`.
return version of '0.0.0' if git describe parsing fails

Until the first annotated tag has been cut in a repository git describe 
fails with the following 128 error:

  fatal: No tags can describe 'SOME_SHA'
leverage `Omnibus::Util#shellout!` mixin for shelling out
add `omnibus project NAME` for generating a skeletal projects
Add `Omnibus.projects` and `Omnibus.project` methods

These helpers aid in common project-related tasks.
add `omnibus clean` command

This command can work at two levels:

* basic clean will remove temporary files generated during the build
  process.
* when the `--purge` option is specified a full clean will be performed
  including removal of the files in the install path AND and
  any generated packages.
improve project load failure error message