Port view helper from Zend View

[alextech]

As requested in https://github.com/zendframework/zend-mvc-plugin-flashmessenger/issues/3 Many thanks to @adamlundrigan for comprehensive checklist of tasks.

Build will most likely fail until Zend View 3 is released to satisfy new composer restrictions that prevent class name collisions. May need feedback on naming convention of PluginFlashMessenger vs. ViewHelperFlashMessenger in import statements.

Notable changes in port are:

• no more differentiating in namespace between V2FlashMessenger and V3FlashMessenger
• implement todo section which requested usage of class constants instead strings now that code is in same repository

[weierophinney]

Build will most likely fail until Zend View 3 is released to satisfy new composer restrictions that prevent class name collisions.

There is no class name collision; this repo uses a different namespace. :smile:

I've got things merged to a local branch, but have one test failure I'm trying to sort out; once that's resolved, I can merge. I'll push any changes back to your branch so that folks can review what made it into the final patch.

[coveralls]

Coverage decreased (-6.7%) to 82.212% when pulling 710a2949ed709cb88a0cd2d27cd4df8c9c223e86 on alextech:feature/viewhelper into 4941d52b7e40af1fe00bc0dd75e09296793ad826 on zendframework:develop.

[weierophinney]

Interestingly, the test failures are dependent on dependency versions. All PHP 5.6 versions, and all "lowest" targets pass, while locked/latest in all PHP 7 versions fail. I'm attempting to isolate the problem now.

[weierophinney]

It's PHPUnit. Evidently, it's persisting changes to superglobals between calls in versions 6 and 7. @runInSeparateProcess does not seem to work, so I'm attempting to find another way to fix the issue.

[weierophinney]

Thanks, @alextech!