* Extract standard token behaviuor to reuse it in other tests

* Add opt in ERC20 migration contract

* Make migration contract not to depend from standard token

* Changes based on feedback

* Improve MigratableERC20 inline documentation

* move behaviors to behaviors directory

* refactor MigratableERC20 into ERC20Migrator

* fix errors

* change expectEvent to support multiple events with same name

* fix tests

* update documentation

* rename MigratableERC20 files to ERC20Migrator

* move to drafts

* test beginMigration

* rename to ERC20Migrator

* missing semicolon  (╯°□°）╯︵ ┻━┻

* add non-zero check

* improve documentation based on review comments

* improve test descriptions

* improve docs

* add getters

* fix contract

* improve tests