PSR-2 compliance

[robbieaverill]

With #219 I've added a separate phpcs ruleset to this module, based on the framework's ruleset at the time with the exclusions removed that weren't actually failing here.

We should be aiming for PSR-2 compliance. The common exclusions in modules are camel caps method and class names since SilverStripe core requires them at times.

Current rules that should be removed easily enough:

<exclude name="Generic.Files.LineLength.TooLong"/>
<exclude name="PSR2.Methods.MethodDeclaration.Underscore"/> (note: semver!)