Use extends rather than regular expressions

More selectors, but better performance
Use @each expansion for downloadable file types

Adds readablity and maintainablity
Space not required after mailto:
background-position: 0 0 is the default value

This is not the case with multiple backgrounds, but that does not apply
to icons
Extend download iterator

Previous version output single classes for each member of the array
Use iterators for sprites

Note to change the sprites to a single column at a later date