Add a DC asset report which generates an Excel file with the full device...

... and rack inventory.

The generated Excel file has the name "DC_Statistics_<timestamp>.xlsx" and contains up to four worksheets, i.e.
- Front Page
- DC Stats <timestamp>
- DC Inventory <timestamp>
- Rack Inventory <timestamp>

The screenshots below show the openDCIM view, and the four worksheets which are generated by the report.

openDCIM
![report_asset_excel_20130925_5](https://f.cloud.github.com/assets/2499906/1212825/e18d2d76-261e-11e3-9abf-dfa3de4b64cd.png)

"Front Page"
![report_asset_excel_20130925_1](https://f.cloud.github.com/assets/2499906/1212823/dc087658-261e-11e3-8bc6-63c99e96b22b.png)

"DC Stats <timestamp>"
![report_asset_excel_20130925_2](https://f.cloud.github.com/assets/2499906/1212832/fa88ed92-261e-11e3-9fc3-8a16daedbebd.png)

"DC Inventory <timestamp>"
![report_asset_excel_20130925_3](https://f.cloud.github.com/assets/2499906/1212836/09ed9aee-261f-11e3-987e-4b4342e2c1f4.png)

"Rack Inventory <timestamp>"
![report_asset_excel_20130925_4](https://f.cloud.github.com/assets/2499906/1212838/12a27fce-261f-11e3-886a-147a88376bb1.png)

An extra configuration value "Page Size" has been added which allows to define the page size on the printout of the Excel file, see below

![report_asset_excel_20130925_6a](https://f.cloud.github.com/assets/2499906/1212863/836a054c-261f-11e3-810c-23384aab36bc.png)