Add resize! methods

Closes #98 if merged.

It took the fast and easy route -- not setting new cells to NA -- seems's consistent with Base, but a little ugly for PDAs:
```
resize!(x, 10)
10-element PooledDataArray{Int64,Uint32,1}:
   1   
   2   
   5   
   5   
   2   
   1   
    NA 
   1   
    NA 
 #undef
```