Added cache control header to enable caching.

[AJMansfield]

Just a small performance improvement.

[azaghal]

Hi, sorry for the late reply, just a bit preoccupied. Glad that someone found the package useful, I was a bit surprised to see the PR for it since the package is a bit stale :)

There are a couple of changes that would be beneficial, though:

1. It might be better to use django.views.decorators.cache.cache_control decorator on the view instead of hard-coding the header. See Django Docs.

2. The max-age should be configurable (maybe calling it PYDENTICON_CACHE_MAX_AGE?), and set to 0 for backwards compatibility.

3. Documentation should be updated to reflect the new option. This should include at least configuration.rst, and quite probably usage.rst as well (to draw user's attention to it since it can affect performance).