Eyeglass deprecation warnings when using `httpRoot` and `assetsHttpPrefix`

[jimmyhchan]

Looks like the eyeglass API changed while broccoli-eyeglass tried to keep the old APIs. The facade seems to not fully use the newer API and deprecation warnings are seen.

• the option httpRoot needs to be handled and moved to the correct place.
• ensure an empty string can be passed as the httpRoot option to override the default / set by eyeglass itself.
• figure out where assetsHttpPrefix is still being set. The option appears to be handled and deleted by broccoli-eyeglass but somehow shows back up such that the deprecation warning still fires
• Add tests exercise the options.

[eyeglass:deprecation] (deprecated in 0.8.0, will be removed in 0.9.0) `httpRoot` should be passed into the eyeglass options rather than the sass options:
  var options = eyeglass({
    /* sassOptions */
    ...
    eyeglass: {
      httpRoot: ...
    }
  });
[eyeglass:deprecation] (deprecated in 0.8.0, will be removed in 0.9.0) `assetsHttpPrefix` has been renamed to `httpPrefix` and should be passed into the eyeglass asset options rather than the sass options:
  var options = eyeglass({
    /* sassOptions */
    ...
    eyeglass: {
      assets: {
        httpPrefix: ...
      }
    }
  });