Retrieval Base Class

[semohr]

I hope this breaks nothing!

New base class adds:

• Fallbacks can be methods/functions or strings. Methods get evaluated and strings are interpreted as filepaths.
• Differentiation between fallbacks and local backups
• Timestamps and update intervals for local cache

A lot of small tweaks and streamlining, also added a lot of documentation. Also enhanced the travis tests for data retrieval.

I tested for a bit and hopefully found all (big) bugs.