Expose getcurrentnet and getbestblock to HTTP POST.

This commit makes the getcurrentnet and getbestblock RPCs available to
clients making HTTP POST requests.

Closes #127.
add getblockchaininfo to btcctl
Improve btcctl handling of empty RPC data returns.

There are several RPCs which do not return any data on success.  btcctl
was improperly treating this as an error condition.