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).
Merge pull request #75 from jarioksa/fix-plot-envfit-labels-vector

Fix labelling envfit plots with a vector of labels
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.
Merge pull request #76 from jarioksa/fix-plot-envfit-labels-vector

Better fix than 83d3bf1 and cca3f27 for resetting labels in envfit
Do not put github README.md to the R package
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.
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.
Fix reconstruction of response matrix in CA (no CCA) for tolerance.cca
Merge pull request #81 from jarioksa/fix-tolerance.cca

Fix reconstruction of response matrix in CA (no CCA) for tolerance.cca
Merge pull request #82 from jarioksa/CCA-must-exist

Support functions for constrained ordination failed cryptically with unconstrained results
fix bug in labels length conditional test
Merge branch 'master' of github.com:vegandevs/vegan