Add support for TAB or ENTER keys in browser.type

Dear splinter team,
I'm using `django-froala-editor` in my projects and want to run behavior tests. As it is a bit tricky to get the `FroalaField` inputs as this are contenteditable DIVs. I'm using the following approach with navigating with TAB keys:
```
import splinter
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.action_chains import ActionChains

with splinter.browser as b:
b.visit("/")
# click on an visible element
self.browser.find_by_id("id_foo").first.click()
# next element is a FroalaFieldInput and is defined by tab order
actions = ActionChains(b.driver)
actions.send_keys(Keys.TAB)
actions.send_keys("AB")
actions.perform()
```

I would like to somehow have a more "natural" possibility to type "TAB" or other special keystrokes, e.g. by passing selenium `Keys` to `browser.type()`. I think as this is implemented in Selenium it could be useful in splinter, too.