PSR-12 ruleset

[geerteltink]

This PR updates dependencies, uses PSR-12 as the base ruleset and refactors the ruleset and test into the PSR-12 documentation format. Some rules had to be left out or replaced as they conflict with the PSR-12 base ruleset.

• Update dependencies
• Import additional rules
• Import array rules
• Import string rules
• Import phpdoc and commenting rules
• Update docs
• Update to phpcs 3.5.1

resolves #18
resolves #19

[weierophinney]

Ping me when you've finished the todo items, as I'd love to review this!

[geerteltink]

@weierophinney and @webimpress PR is ready for review.

[froschdesign]

My review is related to the update of the documentation.

The following problems occur repeatedly:

• the name of the framework is written with space: "Zend Framework"
• for code examples please use always 3 backticks
• for hints in blockquotes please use a headline with the third order as first headline. (> ### …)

I think all these problems can be fixed per search and replace.

[weierophinney]

Left a ton of code suggestions, mostly around proper usage of RFC 2119 verbiage (mostly, I suggest using MUST in place of MAY, as most of the time, these are required rules).

Also noted a few questions around some of the rules that I feel either need clarification, or I need links to discussions about.

[geerteltink]

@weierophinney applied changes as requested, waiting for #22 (comment) and #22 (comment).