CI: create/drop pandas_nosetest db on class instatiation
So I think that the SQL test classes should inherit from: 

https://github.com/pydata/pandas/blob/master/pandas/util/testing.py#L65

then you can add

```
class BaseSQLTestCase(tm.TestCase):

   @classmethod
    def setUpClass(cls):
        super(BaseSQLTestCase, cls).setUpClass()

        # create the ``pandas_nosetest`` db here

   @classmethod
    def tearDownClass(cls):
        super(BaseSQLTestCase, cls).tearDownClass()

        # drop the ``pandas_nosetest`` db here
```

and remove this:
https://github.com/pydata/pandas/blob/master/.travis.yml#L100

This doesn't affect travis, but testing on a static platform (e.g. windows)
where the database in theory could have something in it (if a test was stopped in the middle) and then the next time it runs their is stuff there
