Merge pull request #255 from mrocklin/datamap

Indexing on data layer
Add reduction, column.schema doesn't include name
extract Expr from TableExpr
Explicit reductions
add builtins to compatibility
python reductions
pandas reductions
sqlalchemy reductions
rename .table to .parent
add mean, std, var to table reductions
add expr design doc
standard usage of parent = compute(...) in python
standard use of parent in sql
standard use of parent in pandas
py33 fixes