Adding WebDAV support

[rawdog71]

Include WebDAV functionality to get access to the asset folder

[rawdog71]

Check the Jackrabbit library.

[rawdog71]

After trying to use Jackrabbit and Milton.io, I am going to implement WebDAV support my own way. Seems to work the way I want it to be.
Folders are internally mapped to keywords. So there is only one tier with "folders". Every asset that is mapped to a keyword is in the folder. Assets mapped to several keywords are displayed in every folder. The physically asset is only stored once in the filesystem.

[rawdog71]

https://github.com/ModeShape/modeshape/tree/master/web/modeshape-webdav/src/main/java/org/modeshape/webdav/methods as inspiration