Update gemspec summary and description

* Fix typos
* Make summary and description more specific
Minor improvements to README

* Update introduction to reflect that gem is meant to be used with Rails
Rename gem

Doesn't directly depend on `shoulda-matchers`, so it shouldn't be namespaced by
it. It directly depends on `json-schema`, so namespacing it by `JSON::Schema` is
more reasonable.
Rename to `json-matchers`
Remove `schema_for` in favor of `$ref`

https://github.com/seanpdoyle/json-matchers/issues/4
Address `failure_message_when_negated` missing

In order to integrate with `RSpec`, we need to declare the failure message
methods. Instead of declaring those on the matcher itself, we declare them in
the `rspec` integration file.

This makes future integration with MiniTest easier and cleaner.
Bump to version 0.1.0

Mentions breaking removal of `schema_for` in favor of `$ref`
Improve Failure (When Negated) Messages

Print out both JSON Body and JSON Schema in failure messages
Bump to version `0.2.0`