A storage abstraction for PSK

 - introduce a PskStorage interface which is a simple way to get psk
   for a given identity

 - a default in-memory implementation is provided for test/evaluation
   only, you need to be serious about the way you store your PSK and not
   keep them laying in your serer/client memoery

 - apply dependency inversion principle for pushing the user defined
   PSK storage to the inner handshakers

 - removed harcoded PSKs from Handshaker.java

 - updated examples to show how to provide your own keys using the
   PskStore