Fix for #49 and #98

Attempted to fix issues #49 and #98.

#49 was related to jwindow.data("pnotify") array keeping a reference to all notices ever posted. If history is not enabled, then there is no need for leaking the memory. Fix: unnecessary notices are now removed in pnotify_remove() if history is disabled.

#98 is related to the maxonscreen option check not supporting push: "top" ordering. The check expected oldest notices to always be found at the front of notices_data, but with push: "top" notices are in reverse order in the array.