Use XSalsa20-Poly1305 instead of AES-GCM. #111

...rc/org/briarproject/api/transport/TransportConstants.java

.../src/org/briarproject/crypto/AuthenticatedCipherImpl.java

...core/src/org/briarproject/crypto/CryptoComponentImpl.java

briar-core/src/org/briarproject/crypto/CryptoModule.java

...arproject/crypto/XSalsa20Poly1305AuthenticatedCipher.java

...oject/crypto/XSalsa20Poly1305AuthenticatedCipherTest.java