Add captcha to registration page

Adding a captcha in the registration page to prevent bot registration
Merge pull request #85 from tdurieux/captcha

Add registration captcha
Display cursors of connected users

Add the possibility to display cursors of all connected users.
The cursor position or the selection zone is displayed in the editor and all connected users are listed with their name, current file and current line.
Merge pull request #86 from tdurieux/multi-cursors

Display cursors of connected users
Merge pull request #84 from satabin/wip-entity-components

This merge introduces some compilation warnings which will be removed when updating to scala 2.11.
[imp #88] Add paper name to identify it

When a user creates a paper, he must give a name that is used in paper
list for further reference to it.

Using a name that is not the title extracted from the source is better
for several reasons:
 - Using a simple regular expression to extract the title from the
   source file is not a really robust method,
 - Source title may contain control sequences that are not really user
   friendly to display,
 - Several papers may have similar titles which will make it more
   difficult to distinguish between them.

For all these reasons we found it preferable to ask for a name that the
user can freely chose and edit.