Revert -webkit-font-smoothing to auto

This forces proper subpixel antialiasing which looks way better on Mac. Makes no difference on other platforms.

Jagged lines with `-webkit-font-smoothing: antialised`:
![Jagged lines with -webkit-font-smoothing: antialised](https://cloud.githubusercontent.com/assets/161577/4606672/753760c2-522a-11e4-9569-3cf43310df07.png)

Proper antialiasing with `-webkit-font-smoothing: auto`:
![Proper antialiasing with -webkit-font-smoothing: auto](https://cloud.githubusercontent.com/assets/161577/4606673/78de8e44-522a-11e4-967f-45c0e6ddbc3f.png)