Add pwmantel for pairwise mantel tests

Core algorithm is working. No tests, useful error messages, or coaxing of distance matrix IDs to a compatible order. p-values need to be formatted for the correct number of digits. Still not sure whether to return a DataFrame or some sort of results object that stores a DataFrame with extra metadata. Going with DataFrame for now as it's simpler.
MAINT: Add resource to assets

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ASCII art: http://ascii.co.uk/art/buddha
Merge pull request #417 from ElDeveloper/assets

MAINT: Add resource to assets
Update test_sort.py

deprecated assertEquals changed to assertEqual
Update test_factory.py
Update test_factory.py
Update test_factory.py
Update test_Factory.py
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
Port make_compatible_distance_matrices; add basic tests
Update test_factory.py
Update test_sort.py
using with to open files to avoid ResourceWarning
Format p-values as strings w/ correct num decimals
Update test_factory.py
Update factory_py
Update factory.py
Add tests for N/A p-values and too few permutations
Update skbio/parse/sequences/factory.py