Bump SHKeyValueObserverBlocks 0.2.0

So 

```ruby
s.platform  = :ios, "5.0"
s.platform  = :osx, "10.7"
```

will have the :osx replace :ios (which make sense).
What's the proper way to set both platforms? 

Also, what's the difference between platform and deployment target? 
Are you guys using the same concept as "project" and "target" from xCode?