Unicode mapping, codepoint fixes, and tests!

Hot on the heels of the http://emojisaurus.com launch comes this hot pull request.

Inspired by #35, I added methods to convert name→unicode and unicode→name.

```ruby
>> Emoji.unicode_for("+1")
=> "" [redacted to pass github's PR validation]
>> Emoji.name_for("\u{1F496}")
=> "sparkling_heart"
```

I added a suite of tests, one of which attempts to verify our unicode codepoints. Here's how it looked prior to this PR.

![test-failure](https://f.cloud.github.com/assets/5355/1479587/05c32338-4688-11e3-8ebf-2db461fb4271.png)

I merged in #26 to fix all but one of those. Turns out :stars:'s codepoint was misidentified so I fixed that up too (8e098547ad56825337e7bb5472ac2f1a625ed84f).

![1f303-vs-1f320](https://f.cloud.github.com/assets/5355/1479601/5ad34380-4688-11e3-8748-a94c7774a063.png)

That leaves us with one new failure because :warning:**we're missing an emoji**:warning:

@juliamae, how did you generate the .pngs initially? We need a new one for "NIGHT WITH STARS" `U+1F303`

/cc @josh @trevorturk @asianmack @jonasd