audit: Refresh utilities using code from Mono git master

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.
audit: Add gen-apidiff-html tool to generate apidiff reports

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.
Add an audit.csproj to the MonoDevelop solution
audit: Use dmcs as a compiler for the audit tools

This is what works for me, but you might want to change this for your
system.
build: Output a message when configure checks if we're on Windows
Add audit/data to .gitignore

If you generate audit data that does not belong in source control, put
it in audit/data, it will be ignored by git.
audit: Add a README file