Send btcdconnected notifications without a wallet.

The notified chain server connection state was being passed through
the wallet and then notified to the RPC server, which prevented this
notification from ever firing if a wallet didn't exist yet.  Instead,
make the RPC server register for these notifications directly from the
chain server RPC client.

I'm not happy with this notification and how it's handled in the code,
but to not break existing clients this change is being made.  Fixing
the notifiation mess and modifying existing clients to use a new
notification API will need to be done sometime later.
Save logs to network specific directories.

Fixes #114.
Register for ntfns when the RPCS creates wallets.

Previously, registerations for wallet notifications (new txs, changed
account balances) were only passed up to websocket clients if the
wallet was loaded off disk (SetWallet was called with a non-nil
wallet), and not for the case when the RPC server would create the
wallet (if it wasn't created yet, and the user manually created it
with createencryptedwallet).  This change fixes that by registering
for these notifications when this code path is taken.
Remove a unused channel from chain.NewClient
Refactor Wallet.txToPairs into smaller functions

Also add a unit test for one of them.