Add support for FxCop /ignoregeneratedcode parameter

To ignore generated code when running FxCop, we can now specify the
parameter

    IgnoreGeneratedCode = true

in the parameters for the FxCop task.  The default for this parameter is
`false`, in order to provide backwards compatibility with previous versions of
FAKE.

"_Generated code_" is code which has been decorated with the
`System.CodeDom.Compiler.GeneratedCode` attribute, for example, code
generated when using the Add Service Reference wizard in Visual Studio.