Sainthood?

I see Pope Francis I (What took so long for a pope to take the name "Francis?"  Haven't any of them heard of Francis of Assisi or Francis de Sales, among others?) has given the green light for Pope John Paul (George and Ringo) to attain sainthood.  He's confirmed two "miracles." 

First, what constitutes a "miracle?"  Second, does the proposed saint perform the "miracle" or is it done in his/her name by God?

Are the saints chosen exclusively from Catholics?  Of course, Mother Teresa seemed a logical and deserving candidate for sainthood.  After all, if she wasn't a "saint," what justification can there be for them?  She even donated her Nobel Peace Prize money to the Little Sisters of the Poor.  Now, if that isn't a miracle....

I wondered about this earlier.  What is a miracle?  Who can be a saint?  Do the rules, then, preclude, say, Abraham Lincoln and/or Martin Luther King from sainthood?  Granted, I'm not a Catholic, so it's not my business, but that seems unfortunate (I can't think of the right word).  Who has positively affected people's lives in such wonderful ways as these two?  Can sainthood, then, only be conferred by actions performed after a saint has died?

And, of course, some "saints" have been demoted.  Didn't St. Christopher (with the medals and statues for safe travel) become "Mr. Christopher?"  Has he been re-elevated?  I don't know. 

In some instances, with all of those former popes now saints, it seems like sainthood is like what the schools do in "teacher-of-the-year" awards.  The rewards go to those who play the game best.  Maybe not, but......