Merge pull request #33 from krlmlr/rbuildignore

add travis-tool.sh to package's .Rbuildignore during bootstrap
Merge pull request #32 from krlmlr/6-cran-url

set CRAN URL via environment variable RTRAVIS_CRAN_URL, default http://cran.rstudio.com
use options(repos=) setting for each scriptlet that requires it, avoid writing to Rprofile