Adds support for showing and hiding fragments.

* Proxies <code>FragmentTransaction#show(Fragment)</code> and <code>FragmentTransaction#hide(Fragment)</code>.
* Proxies <code>Fragment#isVisible()</code> and <code>Fragment#isHidden()</code>.
Merge pull request #4 from howaboutwe/fragment-show-hide

Adds support for showing and hiding fragments.