Make getblock return more info about block transactions.

getblock returns block and block's transactions. There is
currently one optional parameter that changes how transaction
details are presented. false (or no parameter) returns only
a list of transactions. If parameter is set to true, getblock
will return some additional transaction details.

These additional transaction details cannot be parsed easily
as transactions are separated by space indentations only.

This patch adds a second parameter (txdetails) that returns
full transaction details in a json form, so that it can be
parsed programatically.

A second parameter is used for backwards compatibility (rather
then changeing first parameter type to int).

No other things are changed, only this block presentation.