Add Gendarme msbuild task

We use the gendarme task to check for potential dispose issues.
Gendarme gets run as part of a command line build. There are
several properties that can be set that influence the behavior:

- verifyfail: wether to fail the build immediately when Gendarme
  identifies problems or report them as errors and continue. Defaults
  to true.
- autoUpdateIgnores: when set to true we add an extra pass of the
  Gendarme tool that comments all places in the *.ignore files that
  are no longer relevant (because the code changed and fixed the
  problem). Defaults to false.
- verifyset: the set of rules to use for checking. Defaults to
  'dispose'.

Change-Id: I394a78a9392a5d0476d813f63d63511e43697f3f