rename build_transaction to build, _build, or create_helper

[sbellem]

a proposal to simplify the interface and remove redundancy

Transaction.build()

appears simpler and less redundant than

Transaction.build_transaction()

Since build_transaction appears to be more of a helper method to support simple_transaction, perhaps we could name it _build() to emphasize it is meant to be used more as an internal helper method.

Also, if we do rename simple_transaction to create as proposed in issue #12 build and create are somewhat synonymous, so perhaps using _build could help in preventing confusion.

Or perhaps, a more explicit name such as create_helper could be better.