jndn

The KeyChain default constructor uses the OS X key chain on OS X, and the file-based private key store on Linux, etc. The default constructor is implemented in NDN-CPP and PyNDN because we have code for using the OS X key chain. But we don't have code for Java. However, to make the API easier to use on Linux, etc. we should implement the KeyChain default constructor, and throw an exception on OS X until we have code for it.