git ignore a few more knitr products
ignore vignettes subdirs in build
rstudio: use devtools+roxygen
db can be a character argument

* also minor doc tweaks
fix missing symbols if plot3dfc called in function

* plot3dfc now uses plot3d.character/plot3d.neuronlist and this causes trouble when symbols in
  expressions are substituted. The safest thing to do seems to be to force evaluation,
  here by using do.call.
* was a bit worried that wrapping the db argument (which can be a huge neuronlist) could
  result in performance issues, but this appears not to be an issue since it is
  not actually modified.
* I tried to use substitute specifying an environment, but I wanted both the scope of the
  current function and the parent environment.
Bump version to 0.5.2