Fixed Tick Placement (Default and Flat Slider)

I've took the advise from spiritdead and fixed my flat slider style and merged his fix for the default slider in.

References: #502, #518

Oh and I changed the progress indicator's color (as you can see in the image below) to the accent brush. I hope that is fine, otherwise I will revert it.

![fixed tickbar](https://f.cloud.github.com/assets/978183/1042721/53a7dfe4-0ff1-11e3-8bad-657ef41a829f.PNG)

MOD EDIT to auto-close associated issues:
Closes #502 
Closes #518 
Closes #519