Refactored the project structure:
- Added new folder 'external/'. There are external libraries used in happah.
	- Added Eigen 3 library files in 'external/Eigen/'. http://eigen.tuxfamily.org
	- Added kdtree++ library files in 'external/kdtree++/'. Modified the namespace. https://alioth.debian.org/projects/libkdtree/
- Removed 'happah/kdtreeLib/' folder .
- Created 'models/' folder for storing 3D models in .OBJ format.
- NearestNeighbor source files moved to 'happah/transformations/mesh_correspondence/'.
- Fixed includes for external libraries.

This refactoring should exclude dependency errors by compilation.