doc: Updated user guide.

- Bumped version number.
- Added section about asynchronous writes.
- Mentioned in scrubbing section that OSDs have to be explicitly set to "removed" before their replicas will be replaced.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3642 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534