Better practices

lib/rack/racer.rb is moved to lib/rack/handler/racer to
comply with its contents. Gem is renamed from racer-rack
to rack-handler-racer to comply with RubyGems guidelines.

Gem specification is updated with new information on author,
gem name and so on. Now the version is the same as Racer's
last checked version.

Note that this release changes license from MIT to ISC. Since
September, 2014 I consistenly use ISC everywhere in projects,
and advice you the same thing.

I dropped RACK_ENV environment variable usage because you
shouldn't rely on environment variables, at all.

http://tinyurl.com/environment-variables-are-bad
http://en.wikipedia.org/wiki/ISC_license