Re-order breakpoints due to cascading.

Because these generate rules for the visual grid, they need to be defined in this order so they cascade correctly. Without this, when on a mobile-sized screen, you'd see 8 visual columns, even though there were only 4 being used.
Clean up span-columns()
Use even padding on table cells
Fix indentation in column specs
Specify comma as seperator for gradient-stops append for libsass
Remove gutter width from last collapsed element

- Fixes #104
Add badges to README
Bump version to 1.4.0
Remove px-to-em as it is in Bourbon
Remove unused test declaration
Rename collpase argument to block-collapse
Add no-display argument to span-columns
Update links section in the README
Add Bourbon tag to SO link
Add license to gemspec
Make shift take parent column argument
Update specs
Bump version to 1.5.0.pre