Add path package

For testing purposes we need to get the long path of filesystem paths
on Windows machines. By default non-UTF8 syscalls will return the short path
(Ex:"C:\PROGRA~1" instead of "C:\Program Files"). This package exports
previously private functions used in the symlink package to determine the
long path of a file/directory.
There is another usecase for this. On more recent version of windows, even cmd.exe will return long paths instead of the old 8 character format (C:\PROGRA~1):
```
C:\Users\Administrator>dir C:\
 Volume in drive C has no label.
 Volume Serial Number is 6E51-CCE9

 Directory of C:\


07/15/2014  02:50 PM    <DIR>          mongo
07/15/2014  06:42 PM    <DIR>          mongo-build
07/15/2014  06:36 PM    <DIR>          OpenSSL-Win32
07/15/2014  06:40 PM    <DIR>          OpenSSL-Win64
07/15/2014  05:44 PM    <DIR>          Openssl-x64
08/06/2014  02:33 PM    <DIR>          Program Files
08/05/2014  02:01 AM    <DIR>          Program Files (x86)
07/15/2014  04:30 PM    <DIR>          Python27
05/03/2014  02:29 AM    <DIR>          Users
08/08/2014  04:04 AM    <DIR>          Windows
```

So if a test will at some point do an exec that will return a path on newer versions of windows, you can be sure that it will be a long format. If ```gc.MkDir()``` will return a short path, we need a way to sanitise both source and destination before comparing. In this case we could either convert everything we test to a short path or to a long path. Converting to long path makes more sense to me.

Here is an example of a test failing because of short/long paths.

```
tools_test.go:57:
    mtimes[tool] = assertLink(tool)
tools_test.go:45:
    c.Assert(target, gc.Equals, jujudPath)
... obtained string = "C:\\Users\\Administrator\\AppData\\Local\\Temp\\1\\gocheck-2775422040480279449\\0\\tools\\1.21-alpha1-win2012hv-amd64\\jujud.exe"
... expected string = "C:\\Users\\ADMINI~1\\AppData\\Local\\Temp\\1\\gocheck-2775422040480279449\\0\\tools\\1.21-alpha1-win2012hv-amd64\\jujud.exe"
```

Any sugestions to work around this without this package would be welcome