Add test and doc for FindBy annotation in page

Could you please test this pull request before I send it to the upstream repo? The idea is to be able to use FluentWebElement in pages even if the name of the field (for example "createButton") doesn't match with the ID or name of the element (for example "create_button"). I realized that this is natively supported by Selenium, although not documented in FluentLenium, so I added a bit of documentation, and added a TestCase just to make sure this actually works.