Detect incomplete records; clean up code

Make use of itertools.izip_longest to iterate over the file 4 lines at a time.
If an incommplete record is found at the end of the file, raise an error.
Add test/data with an incomplete record

Fixes #398
pep8 fix; trailing whitespace
python 3 fix
First low-level API, slow mantel implementation

Tests are passing!
Unit tests for negative and perfect correlation
Add support for zero permutations
Add tests for one-sided greater and less
Add two-sided unit tests
Add a couple more TODO items before I forget...
IDs are optional when constructing a distance matrix.
Refactor to use DistanceMatrix

Slower than it was before due to lots of extra validation/work being done during p-value calcs.
Rename bioenv vegan example datasets

We're going to be adding more vegan example datasets for mantel so need to distinguish between them as they are not exactly the same dataset as what we're using with bioenv.
Move bioenv code into _bioenv module

Decided to go this route instead of keeping it in the base module as this wasn't a great long-term solution -- we'll be adding several more methods to this subpackage and can't put them all in base! 'nosetests --with-doctest' seems to be happy with this naming scheme and it shouldn't affect users.
Add vegan test dataset to unit tests
Add DistanceMatrix.permute method

2D permutation code was previously in mantel but makes sense to have in DistanceMatrix.
Make p-value calculations more numpythonic

Also slightly faster than before.
Add minimum size checking for dms
MAINT/STY: Standardize __future__ imports

The code base had a heterogeneous mixture of __future__ imports where
some modules would import division some would import print_function and
some would import absolute import, with this set of changes all the
imports look like this:

from __future__ import absolute_import, division, print_function

Fixes #351.
Add tests for input without variance