Fix %i and %I literal grammar

This pull request fixes grammar bugs with the `%i` and `%I` literal syntax.

Before, the grammar parsed the literal as a `String` of `String` tokens.

![before](https://cloud.githubusercontent.com/assets/5688/2714831/7eb634b6-c4fe-11e3-8b1a-ddc3eb6c75fc.png)

Now, the grammar parses the literal as an `Array` of `Symbol` tokens, which is what the literal [actually creates](http://stackoverflow.com/a/14532171/1429373).

![after](https://cloud.githubusercontent.com/assets/5688/2714818/457e884c-c4fe-11e3-9f0b-2a133f6f5efc.png)

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NOTE: If accepted, I'm happy to fix up some more bugs I found in the grammar.