spark: into
expr: move min, max to TableExpr, not Column
api: test for into(DataFrame, TableExpr)
add spark and sql imports by default
api: Pretty html printing in IPython notebook
into: NumPy and Iterators
into: bokeh.ColumnDataSource and DataFrames/TableExprs

Goes both ways, preserves column information but doesn't preserve order

cc @chdoig
into: fix repeated test
table serialization
python: support dict.items as sequence in python3

TODO: find the right superclass to use here
Merge pull request #437 from mrocklin/various-fixes-2

Various fixes