Bump version 2.5.0

Correct nullability for tests in to_one

- Use the nullability of the fkey column in question to determine
  nullability for the entire struct to make things easy, otherwise
  we'd have to pluck out one at a time. This makes the tests pass
  instead of fail sporadically.
- Fix volatiletech#160

Bump to 2.4.0

Bump version

Fix modulus fix

Bump version

Add issue template.

Use pg-specific schema tables for fkey detection

- Postgres doesn't care about names for uniqueness of keys unlike mysql
  because internally it keeps "oid" values to keep track of everything.
  Unfortunately this means that the information_schema standard is
  inadequate to differentiate between constraints that are named the
  same (which isn't possible in mysql, but is in pg). Hence we have to
  dip into the pg specific schemas for better or worse.
- Fix naming of the sample schema in the README since it would fail for
  mysql due to duplicate naming.
- Mark test schema up so we don't fix the bad names so we catch
  regressions here.
- Fix volatiletech#85

Fix readme example spacing

Bump to 2.1.3