Fixed tests for version and agent to work on windows

This patch fixes tests for juju/agent and juju/version to work on windows, as well as ubuntu.
Here are a few of the things that had to change:
1) because Windows is not fully POSIX compliant, the file permissions checks have been disabled. os.Chmod() only works reliably on POSIX systems
2) Abstracted some error messages that differ based on OS. For example the "no such file or directory error"
3) used filepath.Join and FromSlash wherever needed
Correct. @trobert2 as this file is new in this branch it only needs 2014. A file's copyright lists all the years in which it was edited