find_link_by_text doesn't like nested tags

Nested tags within link tags cause find_link_by_text and find_link_by_partial_text to fail even when they should. For example, save https://gist.github.com/medwards/8ccbbe698a46ec21e48a to `/tmp`

```
from splinter import Browser
foo = Browser()
foo.visit('file:///tmp/nested-stuff.html')
print foo.find_link_by_text("split")
print foo.find_link_by_text("sp")
print foo.find_link_by_text("sp l it") # this is the output given by foo.find_by_css('table')[1].text
```