Add merge capability for scales

If a specific scale is already present, the new scale will be merged
with the current one instead of completely overriding it

Example:
ggplot(data.frame(x=1:12,y=rnorm(12,50),groups=rep(LETTERS[1:3],each=4)),aes(x,y,group=groups)) + geom_line() + scale_y_continuous(expand=c(0,0)) + scale_y_continuous(breaks=c(50))

This makes it easier to create functions returning full-blown ggplots that you can still manipulate with known scale*-functions