vectorize []-method

Solves issue #11 

Example usage:

```
a <- cdb("mypath", type = "f")
a[] <- MASS::survey

x <- a[]  # get all variables
x <- a[c("Age", "Clap")]  # subset of variables
```

Example with dimensions:

```
a[ , c(2012, 12)] <- MASS::survey
x <- a[ , c(2012, 12)]
x <- a[c("Age", "Clap"), c(2012, 12)]
```