Namespace some packages

A few files are named helper.php, this can cause some issues if not cautious.

Like /plugin/helper.php /user/helper.php. In one of them I used an alias because two helpers were conflicting.

https://github.com/florianv/joomla-platform/blob/PlatformNamespace/libraries/joomla/user/user.php#L15

Here I rewrote the method too since the class name changed :

https://github.com/florianv/joomla-platform/blob/PlatformNamespace/libraries/joomla/utilities/buffer.php#L201

JSession is coupled with the legacy JApplication too... :D I put a comment in the file.

Some loading methods in JTable, JSession will need to be rewrote according the new names but I think we will do that when we consider the tests + autoload ?