Adding `static` keyword to all functions.

Calling a class method without a class instance is only possible if the
method has been declared as static. Otherwise, PHP will generate an
E_STRICT error. This error, however, is non-fatal, and will not stop the
execution.

This commit adds an error handler that triggers a fatal error even for
non-fatal ones. After adding this error handler, all unit tests started
failing.

The fix is trivial: add static keyword to static methods.