Add materials for python errors lesson
Add "case sensitive" and "traceback" to glossary
Finish lesson with item errors, key points
Add cell tags for challenges
Add markdown file for errors lesson
Add note about tabs/spaces
Use better example than numbers from 1-10, since Python allows -1
Add section on IOError
Contribute an addition to the teaching material
Fix layout
Access coordinates of existing elements first
Modifying default layout of bootcamp pages to give instructors and helpers more prominence.

1.  Moved list of instructors from `index.html` to `_layout/bootcamps.html` in order to put it in the header.
2.  Added a dependency in `Makefile` so that the site will rebuild when templates are changed.

Question: should the entire header well be moved into `./index.html` so that it's easier for bootcamp site authors to find and modify?  (We've found in the past that people find the `_includes` mechanism confusing.)
Rephrase according to suggestion
Merging Jess Hamrick's lesson on error messages
Merge pull request #616 from gvwilson/jhamrick-python-errors-lesson

Resolving conflicts in @jhamrick's lesson on Python error messages
Merge pull request #570 from gvwilson/make-instructors-more-prominent

Modifying default layout of bootcamp pages to give instructors and helpers more prominence.
Add YAML header to regex lessons
Merge pull request #618 from r-gaia-cs/regex-jekyll-header

Add YAML header to regex lessons
Adding material originally written by @bct737 on variable expansion in the shell.
Merge pull request #622 from gvwilson/filename-expansion

Adding material originally written by @bct737 on variable expansion in the shell.