drop py2 support

on-behalf-of: @StackStorm-Exchange <info@stackstorm.com>

Add attachment support (#15)

* Add attachment support

* Fix flake8 errors

* Fix flake8 tests

* Refactor attachment classes

* Fix Flake8 errors

Fix incorrect pack reporting if no config

Update version

Change version number

Bump version and document changes

Version bump

Remove deprecated st2actions import

example config

Update version to 0.3.1