Switch from classic context menu to ActionBar in the conversation view.

Just a replacement following the Holo style guidelines. Not implementing multiple selections.

I believe there is another pull request for the multiple-selections version of this, but it is not finished yet. So maybe just go with this "simple" version first.

Here's what it looks like:

![screenshot_2014-03-04-15-52-30](https://f.cloud.github.com/assets/1475672/2322130/118c0252-a3ad-11e3-8a19-9fa3f95380d2.png)