Add support for new rescanfinished notification.

ok @davecgh
Fix unmarshaling of HTTP POST responses.

If connecting to a bitcoin RPC server as an HTTP POST client, full
response objects rather than just the raw result bytes were being
passed to the specific result unmarshalers, causing unmarshal errors
for the incorrect JSON types.  To fix this, first unmarshal the
response body into a rawResponse, and pass only the raw result bytes
(or an error) to the specific handlers.

This was reported by nskelsey on IRC.

ok @davecgh
Log when the initial connection has been made.
Use system CAs when Certificates are not specified.

This commit modifies the TLS setup to only override the RootCAs for the
TLS connection if certificates are specified.  This allows the
Certificates parameter to be ommitted from the connection config to use
the system CAs.
Detect error strings on SumitBlock.

The submitblock RPC returns a string containing an error if it failed for
any reason.  This comment detects a non-null return from submitblock and
converts that string to an error which is returned.
Add missing import.
Unmarshal the SubmitBlock error.

The string needs to be unmarshaled rather than returned directly otherwise
it will have an extra set of quotes plus it's safter to properly unmarshal
it.

Pointed out by @jrick.