Merge pull request #769 from gvwilson/gh-pages

Updating gh-pages from master
Merge pull request #770 from swcarpentry/master

Adition to #769 (Updating gh-pages from master)
Merge pull request #776 from gvwilson/gh-pages

Merging latest changes on master into gh-pages
Comment optional item at YAML header
Merge pull request #777 from r-gaia-cs/optional-yaml-header-element

Comment optional item at YAML header
Merge branch 'gh-pages' of github.com:swcarpentry/bc into gh-pages
Added additional challenge to 04-cond to combine conditional, per discussion with @jdblischak
replaced parameter with argument, identified argument with a defualt value of TRUE, and changed name of argument to use_boxplot
Updated new challenge wording to drop boolean and instead refer to the use_boxplot argument
updated index.md
times for shell, python, git, sql
Merge branch 'section-time-estimates' of github.com:chendaniely/bc into section-time-estimates