Pelican-readtime

* Various convention and code style changes to better conform to pep 8
* Added two additional accessible properties for documents: `readtime_with_seconds` and `readtime_string_with_seconds`
* Slightly increased testing coverage

* Fixed style issues in source files
* Added pull requests 8, 7, 6, & 5

- PR 1: Added light documentation in the form of a README.
- PR 2: Added a new attribute called `readtime_minutes` that will let users just display the amount of minutes instead of minutes AND seconds.