Raise a useful error when trying to activate_session with nil

When trying to perform `ShopifyAPI::Base.activate_session(nil)` instead of getting a cryptic error you'll get the following:

```
irb(main):002:0> ShopifyAPI::Base.activate_session(nil)
ShopifyAPI::Base::InvalidSessionError: Session cannot be nil
	from /Users/csaunders/development/ruby/shopify_api/lib/shopify_api/resources/base.rb:53:in `activate_session'
	from (irb):2
	from /Users/csaunders/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'
```

Please review @alexcoco @awd