JS/jqplot to support horizontal bar charts for better data visualization

[mattab]

Currently we use vertical bar charts in Piwik, which are fine for "Server time" graph and a few others, but really bad for most useful reports such as "websites", "page urls" or "keywords".
The graph in this case is not useful because the legends are overwriting each other, and there is only 6 labels displayed.

Instead we should display horizontal bar graph, plotting for example 15 values, with icons similarly to what we do already with the static graphs:

[[Image(http://demo.piwik.org/index.php?module=API&method=ImageGraph.get&idSite=3&apiModule=UserSettings&apiAction=getBrowser&token_auth=anonymous&graphType=horizontalBar&period=month&date=today&width=500&height=250&truncate=15)]]

Having the JS charts support this new beautiful vizualization would really add a lot of value to Piwik abilities to analyse data efficiently.

refs #3110