[Neo] TranactionUtil into API?

[Peter Neubauer]

Hi folks,
currently, TransactionUtils is in ....impl.transaction, implying that
it shouldn't be exported by e.g. a bundle for use with other clients.
However, in order to efficiently use the API it is very nice to use
this class, so I would suggest to move it into core.api or rename the
util.transaction package to something being part of the API.

/peter