Migration from Zend Framework v2 to v3

Zend Framework v2 to v3 has been intended as an incremental upgrade. We have even made efforts in the past year to provide forwards compatibility features in v2 versions of components, to allow users to prepare their code for upgrade.

This is not a comprehensive migration guide, however. While we know the majority of the areas where breakage can and will occur, we also know that only when developers are actually updating will we see the full situation. As such, treat this as a work in progress, and please feel free to propose updates or changes via issues or pull requests so we can improve!