FlowPyrLK: several fixes + example

- actually use pyramids, it was passing the plain images
- don't reset features on every frame
- methods to reset/set features from outside
- example demoing how to set features using FAST + filtering
  features to a specific area
example-flow-externalfeatures: add project files
Merge pull request #60 from arturoc/feature-flowPyrLK

FlowPyrLK: several fixes + example