Optional implicit waits

This resolves the optional implicit waits portion of Issue #41.  Simply add the following to your test code (e.g. spec_helper.rb or some other place) to enable implicit waits for the has_<element> and has_no_<element> methods in SitePrism.

```ruby
SitePrism.configure do |config|
  config.use_implicit_waits = true
end
```

Note I used the name "use_implicit_waits" instead of "existence_check_should_wait" because it seemed more generic in case it is applied to other queries.