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`