Fix for issue #825 so that dotplot now works with qplot.

This is a fix for issue #825.

It seems that when a dotplot is created using `geom_dotplot` a couple of parameters (binaxis, stackdir, stackgroups, etc.) are assigned default values. When a dotplot is created using  `qplot(mpg, data=mtcars, geom="dotplot")` these params are not defined. The `reparameterise` function of GeomDotplot expect some of these params to be defined thus dotplot does not work with qplot.

The easy fix for this for `reparameterise` to check if these params are defined and if not use defaults. I've implemented these checks and have used the same defaults as used in the `geom_dotplot` function. Using this fix 

`qplot(mpg, data=mtcars, geom="dotplot")`

does not result in an error and gives the same output as

`ggplot(mtcars, aes(x = mpg)) + geom_dotplot()`

https://github.com/hadley/ggplot2/issues/825