Merge branch 'release/0.4.0' into develop

* release/0.4.0:
  buddycloud-server 0.4.0
Remove compatibility with Node 0.6

pg is not compatible with 0.6 anymore, so we don't need to bother
either.
Include systemd service file in package.js

Fixes #107.
Make the "jid" value really clear.
nodejs not node
sync: don't call a queue callback twice

Some time ago async was made a little stricter by forbidding queue and
forEach worker callbacks from being called
twice ([commit](https://github.com/caolan/async/commit/bb43829d29e91c8f314704876293ae3f7b22c82d)),
which absolutely makes sense.

However in some (rare) cases it is possible for the sync progress to do
such a thing: after a subscriptions sync, if a remote node has no local
listeners left, it needs to be purged. But the current code will cause
the callback to be called after this purge as well as after the normal
sync.

This commit fixes this by making sure that the callback can only be
called once.
Travis: use Node 0.10 too
Merge branch 'feature/fix-double-callback' into develop

* feature/fix-double-callback:
  sync: don't call a queue callback twice
Added build instructions
Update README.md
Refactoring SQL query for most recent posts.
Merge pull request #116 from abmargb/develop

Refactor SQL query for most recent posts
New stanza that retrieves all replies of a post.
Add cascading delete to make node deletion much easier
Better SQL upgrade description
Use timezones in PostgreSQL
Better time and timezones handling

Use moment.js instead of isodate.
Sort in the DB, not in JS
RetrieveReplies: newest items first

Most queries reretrieve new items first; do the same for replies.
RetrieveReplies: use items IDs for RSM