callback can be omitted

with a `if` the callback can be omitted, like

    slack.api('chat.postMessage', {text:'hello from nodejs', channel:'#general'});

without the `if` I get this error message

     TypeError: undefined is not a function
         at Request._callback (/user/src/slack-node-sdk/lib/lib/slack.seed.js:87:7)
         at Request.self.callback (/user/src/slack-node-sdk/node_modules/request/request.js:121:22)
         at Request.emit (events.js:98:17)
         at Request.<anonymous> (/user/src/slack-node-sdk/node_modules/request/request.js:978:14)
         at Request.emit (events.js:117:20)
         at IncomingMessage.<anonymous> (/user/src/slack-node-sdk/node_modules/request/request.js:929:12)
         at IncomingMessage.emit (events.js:117:20)
         at _stream_readable.js:929:16
         at process._tickCallback (node.js:419:13)