Authentication

This PR adds all required logic to authenticate users using the API.

To generate a token just send a POST request to `api.myclapboard.com/tokens` with username and password as form fields. API will return token to be used in future requests. Just add that token all request headers as follows:

    apikey: thisIsYourToken

Tests are missing, will be added as soon as Review and Rating tests are fixed and before mergeing `feature/rating-and-review-methods` to master