DOC: update install and run TXM sections

Windows :

• the screenshots have been made with Windows XP ->replace with Windows 10 screenshots
• add the Microsoft install warning screenshots (this software is not certified blah blah blah) OK

Mac OS X :

• explain the “customize” option that allow the user to choose to install R libraries or not OK
• remove the Yes/No install R libraries dialog screenshot OK

Linux :

• the gdebi part is still true, but most of Ubuntu system now use the Software manager to install TXM OK

(from redmine: issue id 1418, created on 2015/07/09 by Matthieu Decorde)