feat(cqrs-backend-http): send command payload as request data

BREAKING CHANGE: commands now use POST method by default.

If you are using GET for commands (e.g. for static reply stubs), you can intercept commands requests and override the default config:

```js
$httpProvider.interceptors.push(function myHttpInterceptor() {
    return {
        request: function(config) {
            if (config.command) {
                config.method = 'GET';
            }
            return config;
        }
    };
});
```