Malformed stylesheet tests

Refs #219, picks up from #358 after some significant refactoring

Here's an initial sketch of a test for malformed color strings in `json.constants` of the default stylesheet, with a streaming fuzzer pipeline to distort data from the default stylesheet to create test fixtures.

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