[Merge] lp:~xnox/upstart/user-log-dir into lp:upstart

[Dmitrijs Ledkovs]

* Documentation is updated
* Checks for absolute paths added (i.e. must start with '/')
* XDG_HOME_* directories & subdirectories are created 

* XDG_RUNTIME_DIR & fallback is not implemented. Need to consult desktop team on fallbacks (e.g. how other XDG compliant software handles it) and then this will be quick to add as a separate merge proposal.

I think this branch is complete for now.
-- 
https://code.launchpad.net/~xnox/upstart/user-log-dir/+merge/143091
Your team Upstart Reviewers is subscribed to branch lp:upstart.