Refactor the library

- Add webpack build system
- Send messages in queue
- Assert that event listeners are cleaned-up (no memory-leaks)
- Expose `Async` suffixed promise versions of the API
- Use promises internally
- Add tests

Add support for comments and blog discussions

Potential fix for setSubscribeCallback

Add failing skipped tests for #14

I don't know what this function is supposed to do; definitely this is
more correct than what's there.

This closes #22.

Initial implementation of API method codegen

Doesn't mess with the exported API. Adds a `lib/api-from-methods` module
exporting all API methods that use the
`this.send(apiName, options, callback)` scheme.

The generated functions work as follows. For every method in
`methods.json`, two functions are generated:

- `camelCaseMethodWith`
- `camelCaseMethod`

The usages are:

```javascript
api.camelCaseMethodWith(options, callback);
```

And:
```javascript
api.camelCaseMethod(param1, param2, callback);
```

`options` currently holds parameters with their names generated from the
source `lib/api.js` variable names. For example:

```javascript
api.getBlockWith({
  blockNum: 1, // <- because `lib/api.js` called the variable `blockNum`
}, function (err, operation) {
  console.log(err, operation);
});

api.getBlock(1, function (err, operation) {
  console.log(err, operation);
});
```

Further improvements could be made to the method data and generated code. Namely:
- It'll require a `json-loader` or way to require `.json` files on
  browser bundlers
- It doesn't handle #22
- When there's no error but the `data.id` mismatches the iterator, the
  callback isn't called; this is a weird design decision

This closes #23.

Lousy macro patch to #20

Fixes errors in `Steem.send` callbacks when `data` isn't
defined. Changes what happens when an error is thrown, since `data`
isn't always passed to API-level callbacks.