Improve JavaDoc

[msgilligan]

The JavaDoc needs work. Writing good JavaDoc for the main RPC wrapper classes will be the focus.

I'm going to try using the AsciiDoctor AsciiDoclet so we can write JavaDoc like this:

/**
 * = JSON-RPC Client for *Bitcoin Core*
 *
 * A strongly-typed wrapper for the Bitcoin JSON-RPC API. *bitcoinj* types are used where appropriate.
 * For example, a block hash can be retrieved as follows:
 *
 * [source,java]
 * --
 * Sha256Hash hash = client.getBlockHash(342650);
 * --
 *
 * `Sha256Hash` is defined in *bitcoinj*.
 *
 * NOTE: This is still a work-in-progress and the API will change. High on the priority list is making
 * better use of https://github.com/FasterXML/jackson[Jackson] to replace some of the current `Map`-based types.
 *
 */

and get output like this.