Add FindsByValue, static ByValue locator

Hrm. I think this might mislead some folks into thinking this was like, an index of something in the DOM or something, as in order of appearance or order in a list, when it is really just referring to an attribute called "index." So maybe we should do, `indexAttribute`? Or just remove this and let clients do `By.attribute("index", "5")`. I actually kind of prefer that. It works for `value` because there's no other real conflict there -- the value attribute is used for element values, and that's standard across HTML input elements.