Added tests for InMemoryStorage.

Review: https://reviews.apache.org/r/19072
Added a --registry_strict_operations flag that must be false for now.

Review: https://reviews.apache.org/r/19135
Implemented optional strictness in the Registrar.

Review: https://reviews.apache.org/r/19085
Switched from LevelDBStorage to InMemoryStorage for registry flag.

Review: https://reviews.apache.org/r/19134
Added structs for Slaves and Frameworks in the Master.

Review: https://reviews.apache.org/r/19166