Add parameter 'day' to clicks_by_day

[ianozsvald]

The clicks_by_day parameter is deprecated but still works and it returns a historic list of clicks per day (unlike, as best I can tell, v3/link/clicks). clicks_by_day accepts a 'days' parameter but this wasn't available in the Python bindings, I've added the option along with 2 new tests.

I've also checked in some pep8 pedantry.

[ianozsvald]

Since this pull request is untouched I'll provide a simple example to monkey-patch the bitly_api with the updated clicks_by_day method: https://github.com/adaptivelab/bitly-historics/blob/master/bitly_api_extended.py