Allow username and password change

**NOTE**: this depends on https://github.com/tidepool-org/platform-client/pull/5

- From "account" page (click on user in top right of navbar), can now change your username (email) and/or password (previously could only change first and last name), thanks to @kentquirk's new `PUT /auth/user` endpoint
- If an error occurs (ex: the email is already tied to an account), show the error message sent back from server
- Moved user API code from `app/tidepool/` (to be deprecated), into `app/core/api.js` and use [platform-client](https://github.com/tidepool-org/platform-client)

![screen shot 2014-04-08 at 4 35 35 pm](https://cloud.githubusercontent.com/assets/1306536/2644848/967751b6-bf2c-11e3-993c-854071644515.png)

![screen shot 2014-04-08 at 4 32 44 pm](https://cloud.githubusercontent.com/assets/1306536/2644852/9ddffb2e-bf2c-11e3-8a13-03a95eea5bc6.png)