Refactor byte manipulation code into sepearte bytes.dart file and add messages.dart with classes for nigori protocol messages

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Remove commented out unused DsaParameters

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Specify default parameters for DSA

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Derive DsaKeyPair from provided secret

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Add bigIntegerToByteArray method to bytes.dart

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>