Set the default context to an empty Hash and update the readme
Allow passing extra context options to nested serializers

Also added documentation to the README.

Note: This assumes that context is a Hash, or at least that
context responds to `#merge(Hash)`, so should be considered
a breaking change for versioning.
Allow failures in 1.8.7 failing specs until JsonApi is fixed.
Merge pull request #14 from shekibobo/feature/better-context

Better Context. Releasing a *minor* version shortly.