permstats: new generic function to extract permutation results

Function extracts the observed "statistic" and permutations results
"simulations" from vegan functions returning permutation or simulation
results for the statistic.

Implemented for adonis, anosim, mantel, mrpp, oecosimu,
permutest.cca, protest. The following permutation methods do not
return permutation results: CCorA, envfit (factorfit, vectorfit),
mso, permutest.betadisper.
add summary.permstat modelled after print.oecosimu

Variable names are still missing, but the current output looks like:

> a <- adonis(dune ~ Management+A1, dune.env)
> summary(permstats(a))
     statistic      z    mean    2.5%     50%  97.5%
[1,]    3.0730 4.5511 1.04294 0.42650 0.94986 2.1514
[2,]    2.7676 2.5055 1.05795 0.21888 0.88788 2.7881
Edit NAMESPACE so much that you can use permstats
rename permstats -> permustats: consistent with permutest
also rename the file permstats.R -> permustats.R
permustats gets the name of the statistic and test direction

Fixes also:  item containing the statistic was wrong in permustats.protest
summary.permustats returns only median and critical size of quantiles

For two-sided that is 0.025, 0.5, 0.975, and for one-sided 0.5, 0.95
or 0.05, 0.95.
add densityplot.permustats
Print only effective limit of quantiles in oecosimu

For two sided tests these are 2.5%, 97.5% and for one sided tests
either 5% or 95%. Always print median (50%), too.
rename: man/density.adonis.Rd -> man/permustats.Rd
Manual page on permustats
permustats returns named statistic instead of "name" item
Pass R CMD check
add density, qqnorm and qqmath methods to permustats
Deprecate direct density and densityplot method for vegan object

Instead use the same methods for permustats objects, and suggest
densityplot() instead of plot(density()).
Move documentation of deprecated density and densityplot to vegan-deprecaed
move deprecated density & densityplot code to R/vegan-deprecated.R
update docs for permustats and depracation of density & densityplot
Update man/permustats.Rd to all new functions
Make 'probs' a user settable argument in summary.permustats