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):
* [ ] fix [glsl-optimizer](https://github.com/kkaefer/glsl-optimizer/tree/amalgamation) linux/clang build errors on travis, then revert 6f112849bebbacedb9c5fed51c349c386ae9738c
```
clang: error: unknown argument: '-fno-tree-vrp'
clang: error: unknown argument: '-fno-tree-sink'
```
Remove these flags or use something like this?
http://bruteforce.gr/bypassing-clang-error-unknown-argument.html
```
ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future
```
* [ ] malformed style JSON
  * [ ] completely wrong structure
  * [x] missing keys
  * [x] unknown/wrong functions
  * [x] malformed colors
* [ ] malformed sprite PNG
* [ ] malformed sprite JSON

/cc @kkaefer