fix the add command to respect the rlimit_* settings

- add special logic for validation rlimit_* keys when used from the
add command that matches the way rimit_* options are processed via
the conf file
- added a test for the rlimit_* validations
allow you to override the virtualenv command to one in your environment
Merge pull request #719 from wynnw/feature/add-rlimit-fixes

Feature/add rlimit fixes