Fixed tests for version and agent
Merge pull request #652 from trobert2/tests

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
Do not allow sending the same metric key more than once in the same add-metric run.
Merge pull request #883 from tasdomas/add-metrics-do-not-allow-key-conflicts

Do not allow sending the same metric key more than once in the same add-metric run.