Turn full error reporting back on in dev.

This got disabled at some point while we
were working on the error pages.
All caching turned off.

production.rb disables caching in production,
fresh_when() calls commented out. Need to leave
this off until we figure out what actually gets
cached and how.
Serve robots.txt from Rack

This allows us to serve different versions
based on an ENV variable. We want robots to
crawl production, but not stage.

Based on this blog post:
http://avandamiri.com/2011/10/11/serving-different-robots-using-rack.html