Support for recording a memo with transactions, …

…likely taken from the payment request that initiated the payment. Use the new SendRequest.memo register.
I'm sure it works for you because you have the source code installed locally. I meant will it do something sensible when the javadocs are rendered as HTML or the source code of the library isn't present. The style used elsewhere is to describe what the property is and the default value on the getter, and describe when you'd want to set it and what to on the setter. Here it's so obvious we don't need much description, but a few examples of what memos might look like would help set expectations.