Improve support for mocked properties

[danielsaidi]

This currently doesn't work:

class MockStore: Mock, Store {

    lazy var hasContentRef = MockReference(hasContent) <-- hasContent is not a function
    lazy var deleteContentRef = MockReference(deleteContent)

    // --> Which requires us to do this:
    var hasContentInvokeCount = 0
    var hasContentReturnValue = false
    public var hasContent: Bool {
        hasContentInvokeCount += 1
        return hasContentReturnValue
    }

    public func deleteContent() {
        invoke(deleteContentRef, args: ())
    }
}