Add support for AsyncConfig & AsyncData

[maxwellhadley]

Non-existent transmitter objects for these packet types are referenced in lib/index.js, causing warning messages to be generated. They need to be implemented, even if only by non-functional stubs

[maxwellhadley]

Dummy transmitter objects now added by PR #101. However it will be difficult to go any further without access to the actual hardware

[TotallyInformation]

Hi Max, getting this on my live instance of Node-RED (only just got round to doing a trace):

(node:10638) Warning: Accessing non-existent property 'AsyncConfig' of module exports inside circular dependency
    at emitCircularRequireWarning (internal/modules/cjs/loader.js:675:11)
    at Object.get (internal/modules/cjs/loader.js:689:5)
    at Object.<anonymous> (/home/home/nrmain2/data/node_modules/rfxcom/lib/index.js:337:43)
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:790:14)
    at Module.require (internal/modules/cjs/loader.js:974:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (/home/home/nrmain2/data/node_modules/rfxcom/index.js:1:18)
(node:10638) Warning: Accessing non-existent property 'AsyncData' of module exports inside circular dependency
    at emitCircularRequireWarning (internal/modules/cjs/loader.js:675:11)
    at Object.get (internal/modules/cjs/loader.js:689:5)
    at Object.<anonymous> (/home/home/nrmain2/data/node_modules/rfxcom/lib/index.js:342:41)
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:790:14)
    at Module.require (internal/modules/cjs/loader.js:974:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (/home/home/nrmain2/data/node_modules/rfxcom/index.js:1:18)

If you wanted to knock up a test version of the Node-RED node with an updated library, I'd be happy to test.