Header cleanup

Hi,

I removed the C++ dependency from the `LevelDB.h` header. I just moved the instance variables to the implementation file which allowed me to remove the `#import` and namespace declaration.

I did a couple of other small cleanups:

- Removed `+librarypath` from the header, it didn't need to be exposed
- Replaced `LevelDB *` return type in the class method with `instancetype`
- Fixed submodule reference and updated to the latest version.