Highcharts: improved support for lists as argument

Changes:

1. add `setListSpec` method
2. add list support for `xAxis` and `yAxis`
3. update `series` to make use of `setListSpec` instead
4. force user to provide data as a list, if not raise an error, see issue #138

However, there might be some confusion with the requirement of using a list. E.g.

Compare:

```
a <- rCharts:::Highcharts$new()
a$chart(type = "column")
a$series(data = as.list(1:3))
a
```

![skarmavbild 2013-06-29 kl 17 20 04](https://f.cloud.github.com/assets/1571893/726091/7723de4c-e0cf-11e2-820b-525bf75c166d.png)

vs.

```
a <- rCharts:::Highcharts$new()
a$chart(type = "column")
a$series(data = list(1:3))
a
```

![skarmavbild 2013-06-29 kl 17 20 15](https://f.cloud.github.com/assets/1571893/726092/7a192d00-e0cf-11e2-8d9f-6e53d38197d6.png)

We could wait to merge the changes for a couple of days, but I put them up here so they are available for discussion. The fourth point might break some code.

Let me know what you think.