read_qvd would be nice to have
Hi,

a lot of data is stored in .qvd files and it would be nice to read a QVD into a Pandas Dataframe, e.g.:

```
import pandas as pd
pd.read_qvd("filename")
```
More information about QVD files:
http://help.qlik.com/en-US/sense/3.2/Subsystems/Hub/Content/Scripting/work-with-QVD-files.htm

I believe to support QVD is difficult, since it is proprietary, but since you managed Excel, I hope it is possible. :)
