[Gendarme] Fix EnsureLocalDisposable rule with properties

Properties can return IDisposable objects, so we need to check
those as well, otherwise we report false positives.
[Gendarme] Add unit test for storing obj in variable

We shouldn't report an error if we temporarily store an object in
a (new) variable and then return that. The Mono 3.2 compiler
produces such code in debug mode.
[Gendarme] Fix problem with returned obj stored in variable

We shouldn't report an error if we temporarily store an object in
a (new) local variable and then return that. The Mono 3.2 compiler
produces such code in debug mode.