Updated blob test to use test accounts.
Renames the BlobFieldStorageService to UserBlobService and cleans up code.

- Separated the class with Jax-RS annotations from the service interface. The annotations
  are not consistently inherited by implementations of the interface, so it's a constant
  annoyance to keep the annotations in sync between the interface and the production/dev/test
  implementations.

  By having a single, final class that carries the anotations and delegates to the service interface
  we only have to define the annotations once.