give unquoted full path string to os.isdir() (resolve #11 and #15)

[gotoh]

os.isdir("..") on c:\temp unexpectedly returns nil. This can be avoided by calling os.isdir() with the full path. This solves issue #15 Additionally, os.getfullpathname() expects its argument to be unquoted, so we must pass the unquoted value. This change also resolves issue #11. .