Remove asserts...

... as I found a case with real data where one of them failed (but vegan
didn't complain). The biplot is virtually the same. Replaced by a comment.
TST: reformat data file
TST: reformat data file
TST: Add testing for RDA scaling 1
TST: Add tests for RDA scaling 2
TST: CCA test (scaling 1, species)
TST: CCA test (scaling 2, species)
BUG: Fix computation of residual site scores
MAINT: More explicit way to assign scores.
TST: CCA test (scaling 1, site scores)
TST: CCA test (scaling 2, site scores).
TST: Ups
Remove unused import

Related to #100.
DOC: Fix typos when citing data.
TST: remove perfect collinearity from test.

We'll need to figure out what (if anything) does R do to deal with
collinearity.
DOC: English... thanks @gregcaporaso!
Merge pull request #108 from Jorge-C/ordination

WIP: Ordination
sample IDs are now just ids

Also removed DistanceMatrix.num_samples as this is just len(dm.ids) or dm.shape[0] or dm.shape[1]. Seemed like an unnecessary method...
add support for construction from a DistanceMatrix
__getitem__ supports ID pair lookup