Pynamodb-attributes

Adding `IntegerAttribute` – same as `NumberAttribute` but whose value is typed as `int` (rather than `float`)

What's Changed
* add IntegerSetAttribute by knap1930 in https://github.com/lyft/pynamodb-attributes/pull/52

New Contributors
* knap1930 made their first contribution in https://github.com/lyft/pynamodb-attributes/pull/52

**Full Changelog**: https://github.com/lyft/pynamodb-attributes/compare/v0.4.0...v0.5.0

UnicodeProtobufEnumAttribute` is like `UnicodeEnumAttribute` but serializes into a Protobuf enum.

Adding `TimedeltaAttribute`, `TimedeltaMsAttribute` and `TimedeltaUsAttribute` which serialize `timedelta` objects into DynamoDB numbers as truncated integers (in seconds, milliseconds and microseconds respectively).