Use shoes-manual gem

This moves to relying on the shoes-manual gem for providing the manual
support. This includes getting the static files out of shoes-core, and
also moving the manual construction code to shoes-manual.
Add --manual option to main Shoes executable
Don't look for shoes-manual by path
Still needed shoes-core/static/downloading.png
Merge pull request #963 from shoes/shoes-manual

Extract shoes-manual as separate gem (and repo)