Initial work
Abstraction for Redis transaction
StateMachine class does not depend on Redis
Moving storage-related classes to the corresponding files
Clumsy abstraction of job queues
Clumsy abstraction of the storage system

TODO:
1. Prepare abstractions for HangFire.Web assembly (by using queries).
2. Ensure abstractions are ready for MS SQL Server
Monitoring API moved to HangFire assembly
All tests passed, but they need to be completely rewritten
Redis storage implementation moved to HangFire.Redis assembly
Starting to implement MS SQL Server storage support
Merge pull request #33 from odinserj/abstract-storage

Abstract storage