Fix +at(slice-name)

Fix the related bug when using +at() mixin with the slice's name instead
of its measure
Fix +at() with last slice-name

Fix previously inserted bug wen using mixin +at('xl')
Rename function -get-index to -get-scale-number

The function -get-index() has been renamed to -get-scale-number(), as it
actually returns the scale number from its name, and not a zero-based
index.
Merge pull request #29 from Djules/master

Fix +at(slice-name)