Fix email encoding

Sending Email in utf-8 was not supported. We are in 2014, and it is a
shame that this is not the default, but we have to live with this. It is
also highly probable that people registering to such a system will have
names containing non-ASCII characters. As it is always better when it is
explicit, setting the email content encoding to utf-8 avoids big
troubles later in production.
[fix #6] Return newly created paper id

When a paper is created, return the paper identifier in the response.
That will allow the client to redirect (or open, or whatever...) the
user to this newly created paper to start working on it.

close #6
[imp #2] Better Java naming of DMP implementation

Do not create anymore a DMP instance for every actor,
but only one object is created in the dispatcher
and handed down to actors on creation.
Fix bug in ResourceDispatcher

On actor creation (first "Join" message), the user was not counted
in the user map.

* Add unit testing of ResourceDispatcher
* Update to akka-osgi/testkit 2.2.1
[imp #2] Fixes & update to last protocol version

* Storage manager is now common: it is created in dispatcher
instead of being instancied in every actors.

* Synchronization handles multiple files:
Until now, a synchronization actor could only handle one file.
It is now possible for an actor to deal with all synchronized resources given
a paper Id. Each document is to be carried in the synchronization message.

* Add some log traces in SyncActor

* Add exception handling on DMP method invocation:
A call to the DMP.diff_fromDelta() method would raise an
InvalidArgumentException if the given delta length
does not match the shadow length.
Fix this by enclosing this call in a Try construction.

* Add PersistPaper message to SyncActor:
Uppon reception of a PersistPaper message, the synchronization actor will save
the content of the in-memory document ('server' representation of the document)
to the disk.

* Add unit tests