Allow merging of data sets via `mi` function

Summary of `mi` objects will output as if they were _complete_ data.frames. Additionally, this should allow subsetting.

E.g.:

``` R
t1 <- turnout[1:1000, ]
t2 <- turnout[1001:2000, ]
t2 <- turnout[500:1000, ]
z <- zelig(vote ~ race + educate, model="logit", data=mi(t1, t2, t3))

# outputs rbind(t1, t2, t3)
summary(z)

# outputs rbind(t1, t2)
summary(z, subset = 1:2)
```
