Wait for url loaded

@natritmeyer 

I implemented implicit waits for the Page.displayed? method because presently it fails immediately if the page has not finished loading yet.  Calling the current version of displayed? results in immediate failure in almost all cases, whether after calling page.load or after performing an action on one page which causes another page to be loaded (via redirect), for example the user clicks a button or link which redirects them to another page.  Take the following example, which will fail immediately 99% of the time when calling displayed?:

```ruby
some_page = SomePage.new
home_page = HomePage.new
some_page.load
some_page.should be_loaded  # First possible failure occurs here.
some_page.home_page_link.click
home_page.should be_loaded # Second failure occurs here pretty much 100% of the time.
```

The new implementation of displayed? adds an implicit wait using Capybara's default timeout value and accepting an optional timeout parameter if a longer timeout is necessary.

Note that I implemented this as an implicit wait in the displayed? method instead of implementing a wait_until_displayed method as is currently in use by SitePrism elements and sections (another pain point to be addressed in a future PR).  This follows Capybara 2.0's discontinuation of wait_until per http://www.elabs.se/blog/53-why-wait_until-was-removed-from-capybara

Please take a look, we would love to get this merged into the SitePrism gem and hope to send at least a couple additional improvements your way.