Merge pull request #13 from bikepit/wheel-building

Added a post about wheel-building class.
Update 2013-10-12-wheel-building-class.md
Merge pull request #16 from bikepit/clarify-free-class

Update 2013-10-12-wheel-building-class.md
Merge pull request #17 from bikepit/master

Clarify that wheel building class is free.