Add support for blueprints

Adding support for the following ambari REST api endpoints:

GET /blueprints => Returns the available blueprints.
 POST /blueprints/:name => Creates a blueprint. 
 GET /clusters/:name?format=blueprint => Export the current cluster layout as a blueprint.
POST /clusters/:name => Creates a cluster.

Summary of additions:

1. adding 'multi_json' gem as a dependency
2. adding blueprint resource to support the GET and POST /blueprints endpoints
3. adding two methods to cluster resource to support above GET and POST /clusters endpoints
4. adding a top-level 'create_cluster_from_blueprint' convenience method

Also adding associated tests and update to README.
This exists already: https://github.com/infochimps-platform/weavr/blob/master/lib/weavr/resource.rb#L44-L46