feat(clients): client registration apis

This adds 4 new routes to manage clients, besides the existing
GET /v1/client/:id. The new routes are:

- GET /v1/clients
- POST /v1/client
- POST /v1/client/:id
- DELETE /v1/client/:id

These 4 routes are protected via the same Bearer token oauth scheme
that the profile server uses. Currently, it's scope is `oauth`.

Besides requiring a bearer token with the above scope, the email
associated with the token must match a whitelist. This whitelist is in
the config as `admin.whitelist`. The default is a regex matching
`@mozilla\.com$` email addresses.

An additional precaution is that these routes are currently disabled
when `config.env === 'prod'`.

fixes #60
Merge pull request #152 from mozilla/register

Implements Client Management Endpoints