Complete refactor with many, many changes
* Removed hard dependency on Skyfield (but can be optionally used)
* Changed default approximation of visible new moon. Now 1 day after conjunction rather than 2 days
* Added calendars for other poleis (Corinth, Delphi, Delos, Sparta) and generation of generic calendars based on several parameters
* Some functions moved to sub-modules
* Documentation moved to sphinx and readthedocs.io