Deserialising XDR base64 Strings to Transaction objects in Java SDK

A few people have asked for this in the last few months on slack and the stackexchange. So, I've just issued a pull request to enable it.

Usage:

Transaction.fromEnvelopeXdrBase64("AAAAAB1V5h7FUsQ ... sfvia6gO");

Great!, we should add it to the C# SDK too (soon!)

And we ported it (https://github.com/elucidsoft/dotnet-stellar-sdk/pull/68)! I know they closed your pull request in favour of another one but you did a great job with it.