Malformed stylesheet tests

Refs #219 

There's some duplication in here between `bin/fuzz-style.js` and `bin/build-style.js` that should likely be consolidated, but here's an initial sketch of a test for malformed color strings in `json.constants` of the default stylesheet. `stylefile.good()` appears to be ?correctly? flagged as `false` with no crashing when proceeding to set a bad style:

```
ASSERT_FALSE(stylefile.good());
std::stringstream stylejson;
stylejson << stylefile.rdbuf();

Style style;
style.loadJSON((const uint8_t *)stylejson.str().c_str());
```

TODO (from https://github.com/mapbox/mapbox-gl-native/issues/219):
* [ ] malformed style JSON
  * [ ] completely wrong structure
  * [ ] missing keys
  * [ ] unknown/wrong functions
  * [x] malformed colors
* [ ] malformed sprite PNG
* [ ] malformed sprite JSON

/cc @kkaefer @mourner