Stop using math/rand.

Use a real PRNG instead.

Fixes #1.
Add a getter for TestDB.name
Merge pull request #2 from metcalf/am-name-getter

Add a getter for TestDB.name
Change testdb API to allow usage with Postgres.

Postgres does not allow selecting a database after a connection is
established. Instead, we can just make the db connection private
to testdb and rely on the caller to create their own connection
to the correct database given the Name.
Merge pull request #3 from metcalf/am-postgres-compat

Change testdb API to allow usage with Postgres.