Docs: change nosetest instruction
Running Ubuntu 12.10.  The recommended way of running tests (http://pandas.pydata.org/pandas-docs/stable/install.html#running-the-test-suite)

```
nosetests pandas
```

did not work.  I had to use:

```
nosetests --exe pandas
```

It would also be useful to know how to avoid the:

```
OperationalError: (1046, 'No database selected')
```

errors.
