Add WebDAV interface to continuum build agent for displaying the working copies

Fixed in r965372 and r965682

added WebDAV interface to continuum build agent

Edited the summary based on the commit comment so the release notes will show what was added.

Can you explain more about the WebDAV interface on the agent? How is it secured?

Made some small changes in r978986

• do not write content as html when dav resource is a collection

Added WebDAV support for the working copies in the build agent

1. there's a WorkingCopyServlet in the build agent that will handle all url pattern: /workingcopy/*
e.g. In order to view the working copy of a project with id 1, the url will be "http://host/workingcopy/[projectId]"
2. support for read only access to working copies
3. can use Sardine as the WebDAV client
4. no security implementation yet.

#4 still needs to be discuss in the dev list. We currently don't have a database in the build agent so we can't use redback for the role-based authorization.