nanoFLANN implementation for KdTree

I made a port of https://code.google.com/p/nanoflann/ for PCL.

Not sure how to add the nanoflann library as a dependency, I copied it on the impl folder as it is just a hpp file. Not sure if it's the proper way to do that.

I didn't do any formal benchmark on it still, the author says that it's faster than the original FLANN but I don't see much difference in my code.

My rationale of doing this port was:
   * doesn't need to build libflann, nanoflann it's just a .hpp file
   * no memory leaks, I found several memoryleaks in the libflann that were hard to debug (I may go back to it and try to fix them)
   * the author says it's faster