Use the Fortuna generator instead of the JVM's SecureRandom. Bug #4.

Note that this is only the generator part of Fortuna, not the
accumulator. The generator requires a seed, which is provided by a
platform-specific implementation of SeedProvider. On Linux the
implementation reads the seed from /dev/urandom.

briar-api/src/org/briarproject/api/crypto/SeedProvider.java

@@ -0,0 +1,12 @@
+package org.briarproject.api.crypto;
+
+/**
+ * Uses a platform-specific source to provide a seed for a pseudo-random
+ * number generator.
+ */
+public interface SeedProvider {
+
+	int SEED_BYTES = 32;
+
+	byte[] getSeed();
+}