BUG: Remove __getslice__ override.

__getitem__ handles slices in pandas.
BUG: Fix read_sql() call

Use keywords calling pandas read_sql() as some of the arguments
changed.
BUG: Use _geometry_column_name in to_json()

Fixes a bug in to_json() when the geometry column is not named
'geometry'.
Merge pull request #90 from jwass/pd_master_fixes

Pd master fixes