Update _media-queries.scss

Generally edited to make the base query include ALL of a class of device, regardless of pixel density.
This allows you to create a query for iPads, regardless of retina or not.
This way set a query for the device, and if you need to you can use specific queries on top of that to target retina devices.

Did the same for iPhone, renaming the base query to "smartphones" since it will match all phones.