Bugfixes: preferences and data storage

A minor bugfix release. It fixes:

• a bug where the empty initial preferences file (serpentime/files/preferences.json) was ignored. This files has been stuffed with some default values for the preferences, making the app less scary to open for the first time.
• a bug where the data folder wasn't created upon installing the package.

First release

The very first release of Serpentime! It features:

• drawing chronodex graph from activities entered in a table.
• navigating through chronodex graphs via a calendar or date navigation bar.
• a preference pane, where the user can set a few options to customise the chronodex graph.