removed vagrant files for release, update README to suggest pre-built
binaries
pandas: By allows series-groupby, repacks to dataframe

Some operations like ``nunique`` can only be performed on series.
We need to internally allow SeriesGroupBy objects during the groupby
operation.  We check this case and recoerce to DataFrame at the end.
sql: add more reverse types
sql: head works on parent of table, not given table
minor edit to README
Merge pull request #334 from mrocklin/various-fixes

Yet another bugfix PR
Merge pull request #327 from talumbau/vagrant_del

Vagrant del
use inheritance to test sqlalchemy types

Otherwise the dialect types aren't checked

TODO, make sure most specific type is chosen
Merge pull request #335 from mrocklin/postgres

use instance for sqlalchemy type checking