merging data.frames columnwise (rbind with different variables, lengths)

Dear List,

I have two dataframes with overlapping colnames and want to merge them. 
  Actually, what I want is more similar to rbind, but the dataframes 
differ in their columns. Here are the examples:

df1 <- data.frame(A = c(1,2), B = c("m","f"), C = c("at home", "away"))
df2 <- data.frame(A = c(2), C = c("at home"))

Here the desired result:

   A B        C
1 1 m  at home
2 2 f     away
3 2 NA at home

Thanks for any help,
Stefan