Set current_path from within Sprockets' context.

All requests to /stylesheets and /javascripts are handled by Sprockets
exclusively. This means a file like /stylesheets/main.css.erb has its
ERb processing handled by Sprockets, not Middleman. We have a method_missing
that forwards references to helpers to the Middleman application, but
current_path wasn't being set because those requests weren't going through
Middleman's endpoint. This change has the Sprockets context explicitly
set current_path to the asset's path before forwarding helper requests
on to Middleman, meaning things that use current_path/current_resource
like relative_assets now have the info they need to work.

Fixes middleman/middleman#764 and middleman/middleman#731.
Never do :debug_assets in the build environment.

Fixes middleman/middleman#754.
Merge pull request #16 from bhollis/debug_assets

Never do :debug_assets in the build environment.
Merge pull request #15 from bhollis/current_path

Set current_path from within Sprockets' context.
Tune up dependencies
Get tests passing again. Scary that I have to fake out the Rails stuff.
Merge pull request #14 from bhollis/master

Get tests working again
try including erubis?
fix stylus support with latest gem
use https gemfile
hacky work around for sprockets adding an extra .js after querystring
test 2.0.0
use full path, not basename, when constructing relative paths
bump
have sprockets add images directory when looping through asset try_paths
map images_dir to Middleman sprockets
add missing append_path for images_dir
added response code step to server_steps
cucumber test coverage for sprockets images
ws