Added missing comments and docstrings to src

Mostly docstrings; some inline comments.
Covers src.execute, src.reader, src.util, src.settings, src.run, and
src.block
Minor code edits: added __call__ attribute to Action because it serves
as a useful explanation of execute(), removed some abstract method defs
that were already being inherited.
Added docstring for salve.py
Merge branch 'documentation/comment-overhaul' into dev