Developer documentation

[peterbrittain]

There are some aspects of the design that should be documented in a little more detail, but they don't naturally fall in the user guide. For example:

1. Internals of Windows/Curses Screens
2. Formal internal API for Effects and Scenes.
3. Frame skipping for refresh (in order to reduce CPU)

This should be fleshed out into a proper developer guide.

[peterbrittain]

Also need to document build environment. There are extra packages in requirements.txt that are not in setup.py. See #73 for what goes wrong without installing them.

[decentral1se]

7ce5d19 does try to add something related.