306 fix cannot read undefined layer

[rafatower]

This does a couple things:

• it makes sure CartoCSS comments are not taken into account further down the line
• it does not attempt to apply turbocarto if there's no layer definition

[rafatower]

Here's a branch testing the integration with Windshaft: CartoDB/Windshaft#692

Note that that test is "more end-to-end" (static rendering of the torque image) and that it crashes without the updated torque dependency.

[simon-contreras-deel]

It seems to be ok.

I am not really sure about filtering comments. I have tested cartocss in Windshaft with some lines commented and works fine. But anyway, it is a commented code, it is not making anything, so I don't see any problem filtering it.