Abstract page implementation for downloading files

This commit adds the abstract class ```AbstractFileDownloadPage```. With this class you can easily add new pages which support the downloading of files - without the need to copy code in child classes.

Attributes are holding the data needed to provide the download (or the inline view for images,..).

I have already adapted the ```AttachmentPage```. As this pull request only affects this mentioned page, I would really appreciate it if you merge this pull request into the WCF 2.0 series. It makes it a lot easier to create extensions which provide the functionality to download files.

Of course I have tested my changes - and I noticed no negative effects.