Add user account update

- Update username, password
- Move all user api calls from `app/tidepool/` into `app/core/api.js`
  and use `tidepool-client`
- Tweak error message from user update api call to show error returned
  from server
- Deduplicate some code in `api.user` to go from Blip's "user" object to
  Tidepool platform's "account" and "profile" objects, and back
Merge pull request #43 from tidepool-org/user-update

Allow username and password change