bundler_ext refactoring

This pull request consists of a few releated patches enhancing bundler_ext. Included are changes

- refactoring bundler_ext into smaller modules for reusability and testing purposes
- removing the dependency on linux_admin as that pulled in additional dependencies which may conflict with the gems being loaded
- adds a BundlerExt#system_setup method analogous to the Bundler#setup that simply activates the system-installed versions of the gemfile gems and adds their dirs to the ruby load path, but does not require them.
- a couple additional changes taking care of a few edge cases

  -Mo