Sorting arrow icon classes are incorrect

[flyingL123]

I think the class on the icons for descending and ascending sorting is incorrect. The library applies the classes fa-sort-desc and fa-sort-asc, but the actual classes from fontawesome are fa-sort-up and fa-sort-down:

https://fontawesome.com/icons?d=gallery&q=sort

[flyingL123]

Here's a PR #110

[kenberkeley]

[kenberkeley]

FontAwesome v4

[kenberkeley]

9b99e43