Reorganize loader for compass support

::Sprockets is the wrong check, since we reference the ::Rails constant directly in the loaded file. This was outright failing when used with an empty compass-sprockets-sass install (without Rails)
Provide image_path that works with Sprockets

Prior to this, functions in sprockets-sass conflicted with ours and caused an infinite loop and StackError. This fixes thomas-mcdonald/bootstrap-sass#257

Note that Sprockets' asset_path requires configuration if it's not already configured by your framework. For example, in a simple Rack app, add the following to your config.ru:

environment.context_class.class_eval do
  def asset_path(path, options = {})
    "/assets/#{path}"
  end
end
Merge pull request #318 from trisweb/tristan_compass_fixes

Fixes for bootstrap-sass used without Rails
bring Changelog up to date
rephrase SCSS file section
Add !default to some font variables missing it.

Fixes #333
Pushing 2.3.1.1
Add `affix` to the list of Javascripts
Merge pull request #335 from brief/patch-1

Add `affix` to the list of Javascripts
Switch back to 2.3.1.0 to avoid confusion due to gem pull...
make it work
Merge pull request #342 from glebm/fix-sprockets

fix sprockets config
Added the _variables.scss file into the project template folder
Added the _variables.scss file to the manifest
Added the @import for the _variables.scss file
Merge pull request #345 from Blackhawkso/master

Made it easier to get to the variables file so can customize the look of bootstrap easier
resolves #356 upgrade to bootstrap 2.3.2
Update to 2.3.1.2
Fixes #366

Fallback for pre rails 4 sprockets (< 2.10.0)
Use correct reference to context through the importer

Fixes #366 in the correct way, hopefully.

I can't cite my sources well, since Sprockets is very difficult to follow... but suffice to say the global options hash sent to the Tilt SASS template engine always has a reference to the Sprockets importer, and the importer always has a reference to the context. Tested with Sprockets 2.2.2 with and without Rails.