Reaplaced os.Symlink and os.Readlink

symlinks are not supported by standard go os package on windows. Replaced all instances of os.Symlink and os.Readlink with the implementation in github.com/juju/utils/symlink. On linux these functions simply call os.Symlink and os.Readlink. On windows they use our implementation
Merge pull request #223 from gabriel-samfira/replace-std-symlink

Reaplaced os.Symlink and os.Readlink

Symlinks are not supported by standard go os package on windows. Replaced all instances of os.Symlink() and os.Readlink() with symlink.New() and symlink.Read(). On linux these functions simply call os.Symlink() and os.Readlink(). On windows they use our implementation