Add top-level convenience functions

[Mr0grog]

Today, you have to create a WaybackClient instance in order to do anything with this library. We should add convenience functions that mirror the search() and get_memento() methods on clients (and whatever other methods might be added to WaybackClient in the future) so you can just call methods without needing to instantiate a client or session:

from datetime import datetime
import wayback

wayback.get_memento('https://nasa.gov/', datetime(2016, 12, 31))

I think these would use ideally use some sort of shared client or session under the hood rather than each call creating and destroying a whole client, although that might cause some issues with leaving some connections open/hanging — needs investigation.

Depends on getting thread safety first in #58.