Timer::stop returns TIMER_NOT_AN_EVENT when timer was stopped
Changed the stop() method to return TIMER_NOT_AN_EVENT when it is given a valid timer event ID.  Given an invalid (out of bounds) ID, it simply returns the same ID that it was given.
Converted the ReadMe file to Markdown, added examples, reference, etc. from Dr. Monk's site.
Minor cosmetic editing, tabs to spaces.
Closes #5.