Add an Objektiv cask (easy browser switching)

tl;dr Objektiv is a menubar app to switch default browsers.

Full description taken from the [website][]:

>Objektiv is a utility that lets you switch your default browser
>easily.  You might find it useful if you're a web designer or use
>multiple browsers in your workflow.

[website]: http://nthloop.github.io/Objektiv/
Merge pull request #752 from ciarand/objektiv

Add an Objektiv cask (easy browser switching)