add -x switch to support running roxygenize() before compilation
-x switch only roxygenizes and exits
do_bind_package also updates files that contain a hard-coded version, in addition to placeholders
roxygenizing also calls do_bind_package

build with alternative workflow:

    rant -v new.version -x path/to/package
    <commit autogenerated files>
    rant path/to/package
support for -V switch to omit update of the version information
remove also .gitignore and .Rproj.user when using -S switch
Merge pull request #1 from krlmlr/roxygen

add -x switch to support running roxygenize() before compilation