The Request packet now contains a list of message IDs, rather than a
bitmap referring to the list of messages IDs in the Offer. This allows
the Request to be understood out of context, e.g. if the Offer and
Request are sent over separate connections or a connection is replayed.

The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

We don't need to call HMAC-SHA-384 more than once to get 256 bits of key
material. The code will throw a RuntimeException if CIPHER_KEY_BYTES is
changed in future so that one iteration's no longer enough.

Fixes bug #25. CMAC is used as the PRF. Note that we're currently using
a version of Spongy Castle with a CMAC implementation that's vulnerable
to a side-channel attack - this has been reported and fixed upstream but
we haven't yet upgraded to the fixed version.

This removes the other projects' dependency on the Android API.