ENH: adding neighbor joining method

This contains initial work on adding neighbor joining. This is a widely used,
basic and "fast" approach for building trees from distance matrices, so is a
core functionality addition. See #420.
ENH: added tests, some reorganization
ENH: commenting, fixed a typo
MAINT: expanded documentation
MAINT: added test of trival case
MAINT: added test
ENH: expanded tests
ENH: expanded tests
ENH: expanded tests
ENH: fix to doc test
MAINT: pep8 fixes
MAINT: pep8 fixes
MAINT: renamed file to avoid name overlap

sphinx doesn't like functions and modules with the same name, so renaming to be
consistent with other modules (e.g., skbio.math.stats.mantel).
ENH: added another independent test of nj
MAINT: fixed typo
Merge pull request #428 from gregcaporaso/issue-420

addition of neighbor joining algorithm