methods/transaction/memo library

notice that get_transaction.dart is tightly coupled to the datastructure of of the electrum server. this has pros and cons, but currently we would only use it to get the memo from a transaction, thus, I added this code to accomplish only that.

Functions

parseAsmForMemo(String asm) → String
https://github.com/moontreeapp/moontree/issues/5 we assume one transaction can only have one OP_RETURN