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