Fixes error when a repos has zero commits.

scheduler caught exception:
undefined method `each' for "":String
lib/github_backend.rb:31:in `block in contributor_stats_by_author'
lib/github_backend.rb:23:in `each'
...
Removed duplicate repos
Larger labels for broken builds
Updated README and screenshot
Better tooltips for leaderboard scoring
Merge pull request #20 from markrickert/fix-no-commit-repos

Fixes error when a repos has zero commits.
Ignore users belonging to org $SKIP_ORG_MEMBERS. This is useful if you are a company with an open project and would like to only view outside contributions
Moved leaderboard defaults into library
Improved "skip org members" feature
Clarified REPOS vs ORGAS usage (fixes #2)
Fixed :skip_orga_members assignment
Fixed "current" period regression in leaderboard
Merge pull request #1 from chillu/master

Awesome UI Improvements