Alias Capybara::Node::Element#set to Capybara::Node::Element#value=

This makes #value act more like a Ruby attr_accessor.

```ruby
find('input').value                #=> ''
find('input').value = 'new value'
find('input').value                #=> 'new value'
```

I wasn't sure if tests were necessary for a one-line method aliasing, please let
me know if they are and I'll add some.

Cheers,
Andrew