Avoid slice out-of-bounds indexing panic.

The gettransaction handler was attempting to lookup the "sent-to"
address of an outgoing transaction from the transaction store (as a
wallet credit).  This is the incorrect address when sending to an
address controlled by another wallet, and panics when there are no
credits (for example, sending to another wallet without any change
address).  Instead, use the first non-change output address is used as
the address of the "send" result.

This fixes the panic reported when debugging issue #91.

While here, fix the category strings used for wallet credits to
support immature and generate (the categories for coinbase outputs).