Removed mixtures of static/non-static members

An important point in the styleguide is to never mix static and non-static members within the same class.

While the code itself worked fine, the problems were made apparent when PHPUnit runs tests concurrently -- it became difficult to understand having state encapsulated within an object for one property when shared amongst all objects for another.

Because of the slight change to API, this release is a minor version increase as opposed to a patch.