change text-max-angle default and doc to degrees

[lbud]

per #92 and related to mapbox/mapbox-gl-js#547
I changed the default to 50 degrees as a less crazy default than 180, but @nickidlugash it would be great if you had any input on what a sensible default might be? /cc @yhahn @ansis

[nickidlugash]

@lbud I'm not sure what a good default might be – 50 deg sounds like a good ballpark (or maybe 45 deg, to be easier for users to envision?), but possibly something like 30 deg might look better. What happens though when it is not possible to place a label within the constraints of text-max-angle? Will the label not show up?

[ansis]

What happens though when it is not possible to place a label within the constraints of text-max-angle? Will the label not show up?

Yes, it won't show up.

[lbud]

@nickidlugash right -- I've been visually checking this out and generally it seems like when you zoom in far enough it tends to be able to find a long enough straight enough segment to place a label.
for example, at 50°:

vs at 0°