add set difference exercise
Created installation instructions for R and RStudio.
Merge pull request #13 from jdblischak/r-setup-instructions

Created installation instructions for R and RStudio.
All badges in one SVG file
Adding duration to DB calculation section
Updating two more Python lessons
Reference guides
Adding links to reference guides to home page
Updated badges

removed unused core badge
added helper badge SVG and PNG
updated the following badge SVG with backported PNG colors:
	instructor
	creator
	organizer
Added learner PNG badge, with colors from old core badge
Updated learner PNG from green/creator color, to blue scheme
Merge branch 'badge-updates' of github.com:ptone/bc into badge-updates
Removing now-obsolete all-in-one badges file
Merge pull request #14 from ptone/badge-updates

Updated badges
Merge branch 'gh-pages' of github.com:swcarpentry/bc into gh-pages
Bringing in new Python material and making old labeling consistent
Adding UBC R materials by Jenny Bryan and others
Importing more testing materials from THW
Adding link to Python testing material
Renaming R material to be consistent with newly-added Pandas material
Renaming 'guide' to 'swc' to make provenance clearer
IPython Notebook for use with NumPy lesson