Rename new transaction register/notifications.

This commit renames the notifyallnewtxs RPC to notifynewtransactions to be
more consistent with the standard RPC names and to more accurately
reflect its intention which is to register for new transactions as they
are accepted to the memory pool.  In addition, the notifications produced
have been renamed to txaccepted and txacceptedverbose depending on whether
or not the verbose flag was set when the client registered to receive the
notifications via notifynewtransactions.

This closes conformal/btcd#98.