Merge pull request #60 from rstudio/bugs/debugging

Roundup of fixes for 0.98 bugs in debugger and environment grid view
Merge pull request #61 from rstudio/bugs/debugging

Fix rsession crash in debugger startup when source ref SEXP is null
Merge pull request #62 from rstudio/bugs/debugging

Avoid use of paste0 (not present in older versions of R)
check whether breakpoints are available (R >= 2.14) at startup
Merge remote-tracking branch 'origin/master' into bugs/debugging
show warning rather than setting a breakpoint if R version is too old