Fixes issue #364 #754  PDF generate empty file on Windows platform - Update report.py

 Fixes issue https://github.com/odoo/odoo/issues/364  https://github.com/odoo/odoo/issues/754
 - PDF generate empty file on Windows platform.

***Impacted versions:***
 - 8.0

***Patch tested on:***

 1. Windows XP SP3
 2. Windows 7 SP1
 3. Windows Server 2003 R2
 4. CentOs 5.10

<strong>All above test conducted using wkhtmltopdf version 0.12.1</strong>


***Current behavior:***

 -  Script detect Windows platform and:
 1. If DPI setting on requested report is set to anything below 96 (Current Odoo Default = 90), script will use 96  (Windows default) instead.
 2.  Avoid deletion of temp files till a successful creation of the required PDF report.