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.
Copy latest versions of mono-api-info.cs and mono-api-diff.cs from Mono
git master, along with associated classes. As a consequence,
mono-api-info.cs now depends on Mono.Cecil.
Do the necessary adaptation for our use case: we do API comparison
between version of our assemblies.
