Added escape character support for BigDataFrame.

This should allow us to deal with messier string data in dataframes.

I also noticed a few missing imports.
So in my test, I get

```ipython
In [4]: "{foo!r}".format(foo='\t')
Out[4]: "'\\t'"
```

which I don't think is the intended behavior.  That's why I manually encoded the `'\\t'` initially.