Make data-transfer related objects serializable.

[tiantianwdy]

• Made EncryptedNumber, EncodedNumber, PaillierContext and PaillierPublicKey and PaillierPrivateKey, StandardEncodingScheme to implement Serializable

• Changed the ciphertext in EncryptedNumber to non-trancient protected final BigInteger ciphertext;

[unzvfu]

Previous related discussion about rejecting the relaxation of access to EncryptedNumber::cipherText here #55.

[unzvfu]

In order to safely serialise an EncryptedNumber you must be certain that it has been "obfuscated" by calling EncryptedNumber::getSafeEncryptedNumber() or EncryptedNumber::obfuscate(). Just adding implements Serializable will not do this. Also keep in mind that the distinction between obfuscated and not-yet-obfuscated EncryptedNumbers is for performance reasons: the obfuscation step is (currently) one of the most expensive parts of the whole system.

[unzvfu]

Also relevant is #3.

[maxott]

Will help to make the library more broadly usable

[hardbyte]

Need to address issue raised by @mhsiah

[unzvfu]

@tiantianwdy Do you really need PaillierPrivateKey to be serialisable? After chatting with @maxott it seems we might be able to avoid making it serialisable (which would be safer) since it's only used in the decrypt method.

[maxott]

Addressed @mhsiah's concern about serialising PrivateKey. The PrivateKey is no longer serialisable and a new 'decrypt` method has been added to the EncryptedNumber to defer private key production to a potentially different mechanism (such a local Keystore)

[gusmith]

Travis build is failing because java 7 does not have the java.util.function package so the code cannot compile:

[error] /home/travis/build/n1analytics/javallier/src/main/java/com/n1analytics/paillier/EncryptedNumber.java:20: package java.util.function does not exist
[error] import java.util.function.Supplier;