Move asset Gzip compression to Sprockets to fix Rails 4 regression

Rails 4 forget to compress assets to `.gz`: https://github.com/rails/sprockets-rails/issues/37.

Rails 3 [compress assets](https://github.com/rails/rails/blob/3-2-stable/actionpack/lib/sprockets/static_compiler.rb#L40) in its `StaticCompiler`, but `sprockets-rails` doesn’t contain `StaticCompiler`, so there is no compression in Rails 4.

I move old `.gz` compression code from Rails 3 directly to Sprockets (instead of Sprockets Rails), because it’s very useful feature and don’t depend on environment (we need `.gz` compression also in Sinatra or in just static site compile like jekyll).