Added the ability to delete user with uid

# Summary

Added the ability to delete a user using their ```uid``` assigned by the consumer of the API. This update only uses the secondary way of deleting a user using this url format:

```DELETE https://api.validic.com/v1/organizations/5176906c6deddac02c000001/users.json```

Also added delete ability to ```Validic::Request``` module.

NOTE: Was unsure how to test user ```user_delete``` method correctly since most specs seem to be pending.