Moved addLinks() to the global Cryptocat object.

In order to develop a desktop client, I have had to make the internal function 'addLinks' public. To do so, I simply moved it into the 'Cryptocat' object. This was necessary to override the onClick behavior of hyperlinks. My experimental desktop client uses Roger Wang's node-webkit as a runtime environment and therefore links should be opened in an external browser by calling `require('nw.gui').Shell.openExternal(url)`.  If you merge this two-line-change into the official version, I'll delete my fork and directly refer to your official repo.