Define all non js-native types on root reference and add  object to define schema of root style document.
Add possible function properties.
Remove color as spec type. Being treated as a 'native' by lint.
Make version an enum -- must be '1'
Layer may match either single or composite layer def.
Adjustments to required properties.
Add tests to validate reference format.
raw => v2.  must be a number.
default-value => default
Look into using v3-style type:'enum'/values:[a,b,c] pairs.
Merge in fixes from v3 testing.
Merge pull request #41 from mapbox/spec-schema

[not ready] Spec schema