clean up errors and warnings generated by "make html"

[earlew]

Doing make html generates a lot of unpleasant warnings and errors. Example:

ERROR: Skipping ./README.md: could not find information about 'title'
WARNING: Empty alt attribute for image color_data_mapping.jpg" title="Mapping between colormap and data in /Users/ewilson2011/git/pyhogs.github.io/content/How to.../colormap_bathy.md
WARNING: Could not process static/post-template.md

It would be nice to get rid off those.