fix logging to file in Windows

**In Windows, Odoo fails to write log messages to a configured logfile**

Impacted versions:
* 8.0

Steps to reproduce:

1. Install Odoo in Windows
2. set `logfile = c:\odoo.log` in openerp-server.conf file
3. Run Odoo in Windows, all log messages are still written to console. 

Expected behavior:

1. Log messages should be written to the configure log file. 

The reason and the fix: 
The reason is that a code error in openrp/netsvc.py. 
The 'FileHandler` class is defined 
in `logging` module, not in `logging.handlers` module.  

The fix is to change the line from 
>handler = logging.handlers.FileHandler(logf)

to
 
> handler = logging.FileHandler(logf)