Make sure that the first pair of neuron in quick-start are most similar

* once again there was some code left over from when we were using distance
  matrices
Add subset.hclust (and merge docs with plot3d.hclust)
Add complete example to hclustfc
fix docs and example for hclustfc
give groups argument a NULL default
don't set default neuronlist on attach - should be up to users to load/set
ensure that data download tests are completely independent

* they were leaving some data behind the session tempdir() which resulted
  in sequential dependence of test runs
no longer test for setting of nat.default.neuronlist
Bump version

* and add news
teach load_fcdata to load rds files as well (and allow specification of target env)
replace 2 broken load_fcdb tests with a real one

* the tests were broken because annotation and fcidtable objects are lazy
  loaded by the package and therefore _always_ available whether or not 
  load_fcdb does anything
Bump version for development
Big speedup for square distance matrices derived from raw scores in ff format

* >20x in my tests
* previously we were doing at least 2x more work, but it was taking much
  longer than 2x. Partially explained by diagonal (which took 1/3 of time)
  but main effect must have been due to slow read of reverse scores
Tests to ensure optimisations for square distance matrices don't break things

* test by fetching scores/distances in square and non-square form
bump version to 0.3.1
change R version requirement to >= 3.0.1

just need a version 3 series for knitr IIRC
plot3dfc: soma positions come from df not somapos

* it made no sense to try to get somapos object since it was not being used
* in any case the soma positions should always be in the attached dataframe