don't switch to the file being debugged unless the executing line number has changed
make environment pane tolerant to null lists of expanded objects
put next button on far left of debug toolbar
tweak build.xml comments
fix console errors when generating arg list for lapply-ed functions
deal with some pathological object types more gracefully
Merge pull request #33 from rstudio/bugs/callstack

fix some debug problems found by the RStudio team
only hide the console interrupt button when a new prompt occurs