Add phpstan and ensure level 7

[tux-rampage]

This will add PHPStan as build check and ensure level 7 as requested in #45

• [ ] Are you fixing a bug?

  • [ ] Detail how the bug is invoked currently.
  • [ ] Detail the original, incorrect behavior.
  • [ ] Detail the new, expected behavior.
  • [ ] Base your feature on the master branch, and submit against that branch.
  • [ ] Add a regression test that demonstrates the bug, and proves the fix.
  • [ ] Add a CHANGELOG.md entry for the fix.

• [ ] Are you creating a new feature?

  • [ ] Why is the new feature needed? What purpose does it serve?
  • [ ] How will users use the new feature?
  • [ ] Base your feature on the develop branch, and submit against that branch.
  • [ ] Add only one feature per pull request; split multiple features over multiple pull requests
  • [ ] Add tests for the new feature.
  • [ ] Add documentation for the new feature.
  • [ ] Add a CHANGELOG.md entry for the new feature.

• [x] Is this related to quality assurance? Adding static analysis to the project's source code will improve its stability.

• [ ] Is this related to documentation?

[tux-rampage]

@Ocramius - Feedback addressed, please check again.