New API Keys for the E-utilities

See https://ncbiinsights.ncbi.nlm.nih.gov/2017/11/02/new-api-keys-for-the-e-utilities/

We need our documentation to mention this and include an example, adding ``api_key='...'`` to an Entrez call.

We may want to register a key for the Biopython test suite?

We may also want to make Biopython a little more cautious on the default limiting of 3 requests/seconds to make sure we don't accidentally exceed this?

Finally, we could relax this to 10 requests/second if an API key is being used.