The gen-apidiff-html tool takes all apidiff files in a directory, merges
them and then applies mono-api.xsl to produce a nice HTML page that
shows the information about the API changes.
The various resources needed by the HTML page (images, CSS, javascript)
are added in the html/ directory.
This is all adapted from an older version of the tools in Mono git
master, under mcs/tools/corcompare.
