Feature/loading api

Fixes #670. Added new API for loading a dataset. For that, I have:

* Modified the Account model, adding a private API key in addition to the public API key.
* Created a method called "validate_request" which tests if the signature provided in the request is the right one, using public/private key. If in the future this home-brew authentication method will be changed for another one (i.e. OAuth), you just need to re-implement the method.