Separated out the tranlucent background/curtain from the dialog, in case you only want to show them at specific breakpoints.  Some of our designs switch between a dialog at small widths and a dropdown/menu style.
Merge Scott’s fixes from last night.
Added an option to conditionally override the scroll position (top position may not be desirable if switching between modal dialog and menu at different breakpoints).
Merge pull request #2 from zachleat/master

Separate dialog content from background