title: xlwings gives "Invalid class string" error
body: <p>I'm simply trying to get xlwings working with:</p>
  
  <pre><code> Windows 8.1
   Python 2.7.5
   Excel 2013
  </code></pre>
  
  <p>xlwings installed OK (it imports without error messages). I've tried using IDLE and also using ipython/Python 2.7.6 and under both I get similar error traces which I don't really follow. Here is the output from the IDLE session:</p>
  
  <pre><code>Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
  Type "copyright", "credits" or "license()" for more information.
  &gt;&gt;&gt; from xlwings import Workbook, Range
  &gt;&gt;&gt; wb = Workbook("C:\Users\Alan\Documents\Xlsaves\Test.xlsx")
  
  Traceback (most recent call last):
  File "&lt;pyshell#1&gt;", line 1, in &lt;module&gt;
  wb = Workbook("C:\Users\Alan\Documents\Xlsaves\Test.xlsx")
  File "C:\Python27\lib\site-packages\xlwings\main.py", line 52, in __init__
  self.xl_app, self.xl_workbook = xlplatform.open_workbook(self.fullname)
  File "C:\Python27\lib\site-packages\xlwings\_xlwindows.py", line 93, in open_workbook
  xl_app = _get_latest_app()
  File "C:\Python27\lib\site-packages\xlwings\_xlwindows.py", line 89, in _get_latest_app
  return dynamic.Dispatch('Excel.Application')
  File "C:\Python27\lib\site-packages\win32com\client\dynamic.py", line 122, in Dispatch
  IDispatch, userName = _GetGoodDispatchAndUserName(IDispatch,userName,clsctx)
  File "C:\Python27\lib\site-packages\win32com\client\dynamic.py", line 114, in
  _GetGoodDispatchAndUserName
  return (_GetGoodDispatch(IDispatch, clsctx), userName)
  File "C:\Python27\lib\site-packages\win32com\client\dynamic.py", line 91, in _GetGoodDispatch
  IDispatch = pythoncom.CoCreateInstance(IDispatch, None, clsctx, pythoncom.IID_IDispatch)
  com_error: (-2147221005, 'Invalid class string', None, None)
  </code></pre>
  
  <p>The results under ipython are similar and also end in a com error. The above seems to indicate that python is finding the xlwings script file. I've tried with excel closed and open; with no excel file specified and with the full file path as above. I'm sure I've omitted something obvious but I don't know what. Hopefully someone can help.</p>