ContourFinder sorts contours by size; can return a fixed number

This is to solve issue kylemcdonald/ofxCv#110 (**Ability to sort ContourFinder results by area**).

- New method `setMaxContours()` allows you to specify the maximum
  number of contours to return.  Since they are now sorted by area,
  this means you will get the *N* largest contours.
- A value less
  than 1 for `maxContours` means that you'll get all contours (same default behaviour
  as before).