Build 404 pages from the theme if they exist
Add MANIFEST.in file

The MANIFEST.in file includes all of the files used by the themes we
ship with MkDocs and excludes __pycache__ (py3) and py[co] (py2).

Fixes #196
Add a 404 page to the MkDocs theme.

This change involved breaking up the context generation to create a sub
context that can be used for rendering pages not based on a specific
Markdown file.
Minor updates after feedback
Merge pull request #194 from d0ugal/404

Build 404 pages from the theme if they exist
Fix: use underscores in google_analytics
Merge pull request #219 from ngzhian/fix216/ga-underscores

Fix: use underscores in google_analytics
Merge pull request #200 from d0ugal/manifest

Add MANIFEST.in file