Make selectedSegmentIndex a publicly readwrite property

To:

* Better mimic a `UISegmentControl` (like #41 did for the property name).
* Simpler syntax when no animation is desired.
Remove no longer used SVSegmentedControlDelegate
No need to keep Demo in podspec
Merge branch 'refs/heads/readwrite_selectedSegmentIndex'
Merge remote-tracking branch 'fork/remove_delegate'
Merge branch 'refs/heads/update_podspec'