skip postgres tests if credentials don't work

These credentials work on travis.ci.
They don't necessarily work on a user's local installation
Merge branch 'postgres-doesnt-fail-if-different-credentials' into pandas-chunking
into: Iterator <- DataFrame
csv: reorder columns in _iter, not in pandas_read_csv

This is nice because we retain the TextReader type information
csv: nicer pre_compute on Head and Projection
csv: use lean_projection in chunking case