Added support for a new GraphiteInstance entity in partial support of issue #134:

Seyren Core:
* Added domain object.
* Added store interface with basic CRUD operations.
* Updated SeyrenConfig to use the GraphiteInstance to encapsulate individual Graphite fields.

Seyren Mongo:
* Updated MongoStore and MongoMapper to support the store interface.

Seyren API:
* Added REST API interface for GraphiteInstance.
* Added bean to implement the interface.

Seyren Acceptance Tests:
* Implemented basic acceptance tests for the GraphiteInstance endpoints.

Haven't implemented anything on the UI yet, as I figure it makes sense to treat all of this as
a "dark launch" until we update the checks to use it.