Post By Email: declare static function

Fixes PHP warnings of non-static method called dynamically

`PHP Strict Standards: call_user_func_array() expects
parameter 1 to be a valid callback, non-static method
Jetpack_Post_By_Email::module_toggle() should not be called statically in
/wp-includes/plugin.php on line 470".`
Merge pull request #564 from jeherve/pbe-static

Post By Email: declare static function