add chunks
add bcolz
numpy: union, not, chunks
into: pass kwargs in type and DataFrame <- tuple
python: distinct works on lists
into: numpy <- list, datetime handling
add bcolz and chunks to __init__
bcolz: use streaming python by default

At least for Selections, By, distinct, rowwise, and nunique

Using NumPy chunks for reductions
add bcolz to testing scripts
bcolz: skip tests if no bcolz
into: error message
into: list <- dict_items (python 3)
Merge pull request #457 from mrocklin/bcolz2

Add BColz and chunking backend