change getPayload to remove dependency on rCharts dimple_layer

I forgot that the only change required to remove the dependency on rCharts `dimple_layer` branch was to change `getPayload` to use `toJSON2` for `params` [here](https://github.com/danielkrizian/rChartsDygraphs/blob/c7a010b029f821fe283c9f998d354e891cd6fd0d/R/Dygraph.R#L40-L42).  Sorry this took me so long to realize.  Please test on your end to confirm this works and resolves #18.