Handle empty clipboard or non-text under Windows

[clach04]

On a newly booted Windows machine clipboard will be empty (or if clipboard explicitly cleared).

Test case:

#!/usr/bin/env python


import os
import sys

import win32clipboard
import win32con

import xerox

win32clipboard.OpenClipboard() 
x = win32clipboard.EmptyClipboard()
# next get will raise; TypeError: Specified clipboard format is not available
win32clipboard.CloseClipboard() 
print type(x)
print repr(x)
print '-' * 65

x = xerox.paste()
print type(x)
print repr(x)

[clach04]

@kennethreitz have you had chance to review this? This resolves a crash. Thanks!

[clach04]

@adityarathod looks like you are the new maintainer for Xerox? This PR addresses a crash on Windows which I believe is mergable without conflict.

I have another feature branch with HTML (Windows only) support if that's of interest :) My main branch is https://github.com/clach04/xerox which has all my changes.