Optimize RDA

* Decouple XDR and ASCII formats by subclassing RDAIO type. 
* Optimize the speed and memory footprint of RDA import by buffering and "vectorizing" column data reads.
 Reading of sample 84M binary RData file on my system before the patch:
```
[~]$ julia -e "using DataFrames; @time read_rda(\"test.RData\")"
 elapsed time: 52.973835777 seconds (5705277252 bytes allocated, 50.75% gc time)
```
  after:
```
[~]$ julia -e "using DataFrames; @time read_rda(\"\test.RData\")"
elapsed time: 15.160802834 seconds (2433811160 bytes allocated, 5.08% gc time)
```
* Support NAs in character vectors.