Add new decorators for providing system user/group

This adds two new decorators that wrap test functions: with_system_group
and with_system_user_and_group. It also renames the with_system_account
to with_system_user.

This will break existing tests, a coordinated pull request in salt will
be made to update the refs to with_system_account.
Merge pull request #14 from terminalmage/add-new-decorators

Add new decorators for providing system user/group