Bcolz loads into memory

If a result is less than a gigabyte then just shove it into a numpy array (which will, in turn, shove things into `DataFrame`s if necessary).

cc @jreback @cpcloud 

Builds off of #721.  So this PR should become a lot simpler once that's merged.
This code is only triggered when your data is a `bcolz.ctable`.  Blaze would help users migrate data between different forms, (`into(bcolz.ctable, 'myfile.csv', rootdir='myfile.bcolz')`) but that's not a decision we're going to make for them.

It's good to remember that Blaze is not a single computational engine.  What we're describing here is just a recipe in a very narrow context.