Item descriptions and more compact match history

Made rendering of match history more horizontally compact, as on some
screens the table stretches very wide.
Also added item description popups in match history.

Removed the min-width property from  #summonerGamesContainer to make it
possible to render the table on smaller screens without horizontal
scrolling.
Added class for identifying table headers for headers in the
summonerGames-table in the match-history.
Added css to make sure that the 'items' and 'date' collumns do not
introduce awkward linebreaks.
Reduced padding on '#summonerGames th' and '#summonerGames td'.

Turned the item-<img>'s into <div><img><ul>...</ul></div> to enable
item-descriptions on the match-history page. Added css-rules for
'table.statistics ul.itemDesc', 'table.statistics .item' and
'table.statistics item:hover .itemDesc' to handle the actual popups.
Added margin-bottom to #summonerGames to prevent page from re-layouting
when displaying vertically long item descriptions (result from the page
becomming less wide with introduction of horizontal scroll bar).