Fixes #7: Caching only disabled in GET requests

This change modifies the behavior of `cache` option by setting the default value dynamically based on request method name. It _disables_ caching for GET requests, while enabling it for POST by default. Documentation has been updated to reflect this change.

Also, the cache-buster parameter has been renamed to `__t` (was `t`) to allow usage of `t` parameter by user.

See #7 for why this could be handy.