build: automatically version package builds, record results in build log, record...

build: automatically version package builds, record results in build log, record all cran2deb generated messages in log.

automatic version works as follows:
- if there is no previous build in the database, use R version with
epoch=0 (will probably change to base_epoch in the DB), revision=1.
- if there is a previous build, and the R version of that build is the
same as the R version of the to-be-built, then increment the revision
by one.
- otherwise use the previous epoch and revision=1 with the new R version.

TODO: grab the output from system()s into the log too.
TODO: whilst version changes make sense, the Debian revision number
probably creeps up a little bit too quickly. No point in versioning
failed builds or repeat builds where nothing changed.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@91 edb9625f-4e0d-4859-8d74-9fd3b1da38cb