ENH: allow MultiIndex column selection from HDFStore
Needs some more testing

badness :)
 @jreback Is this actually more efficient, i.e., does the selection happen _after_ reading in every column? If so, then there's really no point in this PR as I can just select after reading in. What I'm interested in doing is actually reading in only 3 or so columns from about 60; there's about 26M rows, so I don't want to read in 60 columns just to eventually select out 3 of them.
 Just stepping through it looks like things _are_ read in first, but my test only has a single dtype, and my actual use case has 3 or so dtypes, (the 3 columns I want to select have 1 object and the others are either float or int dtype)
 all columns are read (e.g. the entire row), then reindexed. So it won't be more efficient (need a column store for that). 

However, you _can_ do `select_column` for a small number of columns (then concat together). I think their is an issue out their to do that. (in combination with a `select_as_coordindates` which creates the row indexer that you pass as the `where`). So on a wide table this would be MUCH more efficient.
 Blah okay :) I'll see if I can't work something up for that issue.
 though this _does_ enhance the selection syntax a bit :)
 @cpcloud can you update? forgot what this was for
 closing, but feel free to revisit
