Modernize the package and finalize QA tooling

[weierophinney]

This patch modernizes the package to follow guidelines as set in our maintainers repository.

In particular:

• Moves documentation from /doc/ to /docs/.
• Adds contribution and support docs.
• Normalizes the composer.json structure.
• Normalizes various package files (.gitignore, .gitattributes, etc.)
• Ensures the Travis configuration uses up-to-date practices with regards to lowest/locked/latest strategy.

Additionally, this patch incorporates commits from #141 that accomplish the following:

• Provides a polyfill for the PatternPluginManager to ensure it works with both v2 and v3 versions of zend-servicemanager, fixing an issue with PHP 7.2 compatibility.
• Adds PHPUnit 7 as a potential target for running unit tests.
• Provides a number of features around enabling extensions that act as cache backends.
• Drops HHVM support.

[weierophinney]

Pinging @webimpress ...

[weierophinney]

Initial test runs that have actually completed (a number had segfaults and needed to be restarted) have been mostly successful, with one specific failure: the Cache\IntegrationTests\CachePoolTest::testHasItemReturnsFalseWhenDeferredItemIsExpired test case often fails. I suspect this is a timing issue primarily.

[weierophinney]

I honestly think at this point the only way we'll get tests to run completely is to separate the adapters into their own repos, as @marc-mabe has planned for some time.

[michalbundyra]

@weierophinney

Pinging @webimpress ...

Sorry, I was not able to have a look before... LGTM 👍 😄

[thomasvargiu]

@weierophinney About Cache\IntegrationTests\CachePoolTest::testHasItemReturnsFalseWhenDeferredItemIsExpired failed test, it's normal.

The Zend\Cache\Psr\CacheItemPoolAdapter adapter doesn't support driver deferred saves, so cache items are saved on destruct or on explicit commit() call.

I'm working on it and I'll open a PR to skip that test in a while.

[thomasvargiu]

Open #149