Added "most recent N builds" limiting option for coverage graph.

Hi,

I wrote this patch because the generation of coverage graphs is very slow (unusable, never finishes and eats cpu), because of the huge size of our project (40MB coverage xml file).

I realize that the real fix for this speed issue would need some kind of preprocessing of xml files into a database or a quickly parseable, indexed binary format. This is just a workaround which enables me to limit the number of builds taken into account. It could even have use cases if the speed wasn't an issue.