[CocoaPods] Update to CocoaPods 0.36 plugin API

## :warning: *do NOT merge -- Awaiting CP 0.36 release * :warning: 

From CocoaPods 0.36 you will need to explicitly mark which plugins you want in your Podfile. So you will need something like the following in your Podfile.

```ruby
plugin 'slather'
```

Re: #29 /cc @neonichu 

---

@marklarr This also means we can provide metadata from the Podfile to the plugin hook, if we need to provide any configuration.

```ruby
plugin 'slather', :foo => :bar
```