v5 Caching

[speatzle]

Support Caching of Commonly needed information like Metadatakeys and ResourceTypes.

Related to #30 since this should also cache the Session key Cache locally.

Open questions:

• Should this Caching be done in the API client or in the helper?
• Global caching Settings for the client?
• Ways for bypassing the cache in the client?
• Signature changes required?
• Should potential signature changes be done with together with #65 ?

To prevent Signature changes and waiting for #65 we could add caching to only mostly static things(ResourceTypes, Metadatakeys, Sessionkeycache...) and provide a way for clearing the entire/parts of the Cache.

Login should flush the cache.