Add an optional DNS caching mode

[François Marier]

It was suggested that this library should be caching the response of the DNS queries for a bit:

  https://bitbucket.org/zzzeek/tracvatar/issue/1/support-for-federated-avatars-using

It could use a contextual object like this:

  ctx = pylibravatar.Context(cache=True)
  avatar_url = ctx.url("<email address hidden>", https=True)

and let applications control the duration of the caching.

The contents of the cache would be a mapping from domain name to avatar server.

The first step of course would be to measure whether or not this has a significant impact on a page that runs libravatar_url() over 100 different emails (where 60% should be using gmail.com, yahoo.com or hotmail.com).