adds basic functionality graphs and trees
adds MST
adds graphs tests
adds __str__
adds PointTree
Change to British spelling

Change center to centre
Change color to colour
Fix some docstrings

Major changes happen in PCA, keyword arg center becomes centre
Move extract_pathes_local to image

Changes to a more efficient Cython version as well
adds relative locations
Merge pull request #479 from patricksnape/extract_patches_on_image

Extract patches on image
Merge pull request #480 from nontas/graphs

Graphs functionality