Support pg-promise

[danvk]

The Queryable interface assumes you're using node-postgres, i.e. the pg package. But it shouldn't be too hard to support pg-promise as well, or at least to include an example of a wrapper.

From my testing, there were a few oddities with pg-promise, for example selects with ANY clauses and UUID types didn't work.

For reference:

• pg has ~3M downloads/week
• pg-promise has ~300k downloads/week