Feature/#3 kv detail view controller for types

[brototyp]

Thanks for your pull request. There is one point of the concept I would like to talk about. That is the process of registering VC-Classes for certain value types. I am not sure if this is the best way. Downsides I see this was:

• The user has to register even the given ViewControllers (Bool, Number, String)
• The registration and the mapping is implicit. This way it isn't possible, or not clear if it is possible to override the VC for NSStrings. You cannot directly see the mapping.

How about something like this:

• We auto-register all the given VCs
• Registering another VC is always a combination between the VC and the Type. If there is already a match, it will replace it. Thus: If the user chooses to set a VC for NSStrings we will replace ours with the user-selected.

[bernhard-eiling]

I like the introduction of the protocol describing our DetailViewControllers. This also serves the readability of our API for the KVDebugger.

Also I like the emphasis of an optional saveBlock and descriptionString by removing the setting of those properties from initialization to their setters.

Overall nice additions and code style refinements!