Making CBMDictionary explicitly internal

[philips77]

This PR changes the access modifier of the new CBMDictionary class to internal.
The class was added in #115.

[nathandud]

I had to double check but the Apple Docs specify that internal is the default access control level if not specified. However, I looked over the rest of the internal classes (CBMCharacteristicNative,
CBMDescriptorNative) and since they are all explicit about being internal, I think it's the correct move here.

[philips77]

Yes, you're right. I forgot. And that's also why I try always to write the modifier explicitly. In Kotlin the default is public, so it's better to always write it.