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.
Plug on Rails to tell it to precompile our own images

Rails changed the way we automatically precompile non-javascript/css files
to make easier to third-party plugins to choose which files they want to
precompile instead of precompiling everything under vendor or lib.
Fix typo

Thanks @cgag

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