Windows Xenstore Support

Hi,

This branch allows PyXS to work on Windows in combination with the GPLPV drivers.  The tested environment is:
Windows 2008R2
GPLPV 0.11.0.3
Python 2.7.6
pywin32-218.win-amd64-py2.7

The modified code was successfully tested on Windows and Linux with:
```python
import os
import pyxs

if os.name in ["nt"]:
    xs = pyxs.Client(None)
elif os.name in ["posix"]:
    xs = pyxs.Client(xen_bus_path="/proc/xen/xenbus")

print xs.ls("/tools/data/vminfo")
print xs.read("vm")```