Add tests to check if Grokily exposes a list of available tenses for a given language.
Fix tests just added to check that Grokily exposes a list of tenses - these were firing incorrectly.
Add to_s and to_hash methods to tense objects to make them easy to expose later on.
Expose a list of supported tenses through /language/tenses or /languages/tenses.json. Update the README file to reflect this.