Fix missing filename in Sprockets filter's error message

Now uses two variable names so the original value isn't clobbered by AssetScanner's return value.
Make includes with theme syntax work in Sprockets filter.

Passes 'theme' setting to AssetScanner instance, so the following syntax will now work:

    //= require "theme:js/ticker.js"

Plugin syntax already works:

    //= require "plugin:Bootstrap:components/jquery/jquery.js"
Fix undefined index warning.

Forgot about cases where there is no 'theme' index in the settings array.
Adding theme/plugin inclusion tests for Sprockets filter.
Some code sniffer fixes.
Merge pull request #179 from deizel/patch-1

Theme support for Sprockets filter.