don't restrict tests only to master branch; instead test all branches that don't end with -expt
Make the `.Rbuildignore` fanciness work with `set -e`.

I stupidly forgot that `$?` isn't something you can check with `set -e` -- if
it's on, you die before you check the previous result.
Merge pull request #43 from krlmlr/except-expt

don't restrict tests only to master branch; instead test all branches that don't end with -expt
comment on the restriction of tested branches
Merge pull request #45 from krlmlr/except-expt

comment on the restriction of tested branches