Make DTLS layer work without CoAP

Hi,

This patchset makes it possible to use the DTLS layer with other protocols than CoAP. Now a user of DTLSLayer can provide a class which implements a Message interface to represent a message. One option is to use the CoapMessage or someone could use a object storing a raw message.

I used this to test tinydtls with the echo service in the tinydtls test directory.

Hauke