Use "db" prefix for dtypes

[tswast]

When we do upstream this to panda (#30) I suspect there will be times when this package is still used alongside the new pandas.

For example, to support INTERVAL data type, I suspect we'll want to add it here first before moving it upstream. There is a DateOffset class we'll want to use, but I don't believe it's a full dtype.

Even if that's not the case, I don't want to cause conflicts if this package happens to be installed an imported somewhere.