Set explicit caching everywhere.

This explicity sets the caching behavior for all goodies. Goodies which
did not have caching behavior set in their package have had it added.
Similarly for the tests.

This resulted in a number of changes for caching behavior, mostly
moving towards more frequent caching.

I identified three cases where goodies should not be cached. They are when
the result depends upon:

- some random element,
- the time of day or year (as with relative dates), or
- the location of the requesting user

In all other cases, I presumed that the goodies were sufficiently
deterministic in their output to warrant caching.
For future reference:

This is actually `is_cached => 0` in the `return`; not `{is_cached => 0}` as indicated in https://github.com/duckduckgo/zeroclickinfo-goodies/pull/655#discussion_r18172282

A similar directive is added to `test_zci` in the test files in order to make sure caching is turned off when appropriate.