build: add parsing of version info from version.txt file
This makes it possible to have proper version numbers in the build
when the source tree does not come from svn (e.g. a tarball).

Also, automatically adds version.txt to our source tar that is
created with "make pkg.create".


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6005 b64f7644-9d1e-0410-96f1-a4d463321fa5