Harris features(corner) and Szeliski features added.

Harris corner features and Szeliski features added.

    i = Image("bolt.png")
    h_features = i.findFeatures("harris", threshold=500)
    for feature in h_features:
        feature.draw()
    i.show()

    
![boltharris_corners](https://f.cloud.github.com/assets/1256649/132095/7b557ede-7078-11e2-8861-fcf0aa694758.png)

----------

    i = Image("bolt.png")
    h_features = i.findFeatures(threshold=500) #szeliski
    for feature in h_features:
        feature.draw()
    i.show()    

![boltszeliski_img](https://f.cloud.github.com/assets/1256649/132104/b0307758-7078-11e2-9357-667d2ac79b6a.png)

----------

Harris, threshold=500

![mtestharris_corners](https://f.cloud.github.com/assets/1256649/132105/c791a02a-7078-11e2-9c6a-a7927592e049.png)


Szeliski, threshold=500

![mtestszeliski_img](https://f.cloud.github.com/assets/1256649/132107/d8b529f8-7078-11e2-91bd-095e27345032.png)

-----------

Harris, threshold = 5000

![corner_sampleharris_corners](https://f.cloud.github.com/assets/1256649/132108/e7cc4c50-7078-11e2-820f-ff9929b1b040.png)


Szeliski, threshold = 5000

![corner_sampleszeliski_img](https://f.cloud.github.com/assets/1256649/132112/fac10044-7078-11e2-92d4-66291a969b9b.png)

Tests are included.
Future work: Including `Tomasi` and `triggs` features.