Move the hooks to generate tokens in opensearch_token.module

As the search tokens implemented by the module are not specific to the module but they are search tokens, it makes more sense to implement them in a module different from opensearch.module.

In this way

1. The main module doesn't depend on token.module; users of the actual version of the module would update without any troubles because of the additional dependency the module would set.
2. Modules could use the search tokens without to force users to enable a module that they don't require.
3. In the case search.module implements tokens in Drupal 7 (or there are different modules implementing such tokens in Drupal 7), the token module would not be ported, but still it would be possible to port the main module.