Fix labelling envfit plots with a vector of labels

Fixes bug report #74 at github.com: it was documented that you
do not need to use a list(vectors=, factors=) to relabel plotted
items if there is only one type of items (vectors or factors).
Better fix than 83d3bf1 and cca3f27 for resetting labels in envfit

Previous fix only looked at the null of internal labels, but these
can be null because the item (vectors or factors) was missing, or
the item was there but names were missing. Fix cca3f27 failed
in the first case and 83d3bf1 in latter case. Now we check for
the existence of factors/vectors, and if only one them is present,
we reset it labels whether they existed or were null.
Update DATE for the last change in the released version
vegan 2.2-0 was released: everything goes to 2.2-1 from now on
Do not put github README.md to the R package

(cherry picked from commit 06f649426a44e065c5e435bbdfaf4d113ece9a22)
Fail informatively if support functions for CCA are used with CA

Several cca/rda/capscale support functions only worked with
constrained ordination, but did not check their input. This resulted
in cryptic error messages such as

Error in chol2inv(Q$qr, size = rank) :
  'size' argument must be a positive integer

Now they fail with more informative error message that tells that
the method can be used only with constrained models.
(cherry picked from commit 4d98639b1cd10a84ed41182a6e7123b84315e441)
More comprehensible error message when user asks biplots of CA

There is no biplot for CA, and its results were handled by
biplot.default that gave weird error messages. Now biplot.cca() was
added to give an informative error message. An alternative to this
would be to rename biplot.rda to biplot.cca: it already has an
error message for non-rda objects, and would only handle rda.
(cherry picked from commit 543ead1092a37e5791408bfc771d66a9f89efe28)
Fix reconstruction of response matrix in CA (no CCA) for tolerance.cca
(cherry picked from commit 0434ac4e653aeccb9b3f903a30a9fdfa55f6d7ee)