`StaleElementReferenceException` added to `WebDriverWait`

I've noticed that sometimes [`ElementGetter._get_element`](https://github.com/alisaifee/holmium.core/blob/6d64eadc96fa7e24ac092139cef1dbdf4eb47246/holmium/core/pageobject.py#L276) throws [`selenium.common.exceptions.StaleElementReferenceException`](http://docs.seleniumhq.org/exceptions/stale_element_reference.jsp) exception while waiting for collection of elements which is dynamically changed. 

It's normal `selenium`'s behavior to raise it in such cases and one of common ways to deal with it is just [ignoring it](http://engineeringquality.blogspot.ru/2013/08/ways-of-dealing-with.html). So, I put it into ignore-list.