Added getter for PageFactory

I need Pages Object to belong to two different websites and its classes sit on separate namespaces. So I need to be able to overwrite the namespace on @BeforeScenario.

Since pageFactory is a private property I had no way to redefine it on runtime, to circumvent that I added a getter so I can change a re-set it after.

Usage example
````php
    /**
     * @BeforeScenario @alice
     */
    public function visitAlice()
    {
        $pageFactory = $this->getPageFactory();
        $pageFactory->setPageNamespace(static::PAGE_NAMESPACE_ALICE);
        $this->setPageFactory($pageFactory);

        /** @var \Behat\Context\Shared\NavigationContext $navigationContext */
        $navigationContext = $this->getSubcontext('NavigationContext');
        $navigationContext->visitSite("Alice");
    }

    /**
     * @BeforeScenario @bob
     */
    public function visitBob()
    {
        $pageFactory = $this->getPageFactory();
        $pageFactory->setPageNamespace(static::PAGE_NAMESPACE_BOB);
        $this->setPageFactory($pageFactory);

        /** @var \Behat\Context\Shared\NavigationContext $navigationContext */
        $navigationContext = $this->getSubcontext('NavigationContext');
        $navigationContext->visitSite("Bob");
    }
````