Compliance with the new API-key policy.

[Demetrio92]

Google changed their policy, and keyless queries are not allowed. However, given a valid credit card it's possible to set up a free account. Google swears not to charge anything as long as the status is not changed manually to "payed".

TODO:

• Throw an error on an attempt to use without an API key.
• Add an elaborate explanation to the readme
• add a message that should be triggered on package load