Added .DS_Store to .gitignore
Merge remote-tracking branch 'upstream/master'
Add robots.txt entries for provider's crawlers

Google recommends setting an empty robots.txt Disallow for their user
agent, Mediapartners-Google for sites serving their ads.

To prevent users from having to add these entries on their own, we can
add them automatically. Each provider now has a $crawler_user_agent
parameter, and the Disallows are filterable.
Updated changelog and stable tag
Merge pull request #76 from nickdaugherty/master

Add robots.txt entries for ad provider's crawler automatically, if applicable.