expr: Reductions given more explicit names and dtypes

Rules like encoded in the common reductions

sum(dtype) :: dtype
count(anything) :: int

Additionally the names of the reduction are now appended onto the dshape, e.g.

    >>> t['amount'].sum().dshape
    {amount_sum: float64}
data: CSV supports missing values by defalt
python: head of large numbers doesn't evaluate tuple

remains a lazy iterator
csv: tests respect option types
Merge pull request #400 from mrocklin/reduction-dshape

Reduction dshape and csv missing values