Implement list.clips

Hiya! I'm creating a quick & simple mobile app for getting "inbox zero" on Kippt using my iPad on the bus. I decided to use your gem, but getting the contents of inbox was kinda difficult. It is more than likely, that I just didn't understand the documentation. Sometimes I have the dumb. Anyhow, I made a fork and quickly created a new method for Kippt::List so that it'd work the way I assumed it to :) And here it is:

```ruby
client = Kippt::Client.new(username: "vesan", token: "2544d6bfddf5893ec8617")
list_id = 10
list = Kippt::List.new({ id: list_id }, client)
list.clips # Returns a Kippt::ClipCollection
```

Disclaimer: I have never used RSpec, so I have ___no idea___ how good the unit test is. I just cargo-culted it to fail and then pass. Pretty much what I did with the actual implementation (cargo-culted from clips.rb, tsihihii :smile:).

Cheers for the gem! :)

– ajk

PS. RSpec has some spectacular debug info for failing tests. Huh.