Support xPath for Windows pathnames
This PR implements a string representation of `xPath`, which is valid for local paths (also windows) and remote URLs.

Additionally, some `os.path` methods are fixed for remote URLs on Windows machines.

Now, on Windows machines:
```python
In [2]: str(xPath("C:\\dir\\file.txt"))
Out[2]: 'C:\\dir\\file.txt'
In [3]: str(xPath("http://domain.com/file.txt"))
Out[3]: 'http://domain.com/file.txt'
```