added kdtree support and example

using the same approach as in the rest of the wrapper, wrapped
`pcl::KdTreeFLANN` from `pcl/kdtree/kdtree_flann.h`. As in other cases,
appended a `make_kdtree_flann` function to the `PointCloud` class.
At this stage, the only method exposed is `nearestKSearch`,
in particular the version which takes a `PointCloud` and an index into
it to choose the points for comparison:
http://bit.ly/WWI9JS

Further added a convienience method which finds the k nearest
distances for all points in the provided point cloud. For an example,
see `examples/kdtree.py`