Add secondary caption field to multi-up, carousel, and item+text blocks.

Closed #393 

I have a few questions that @ggeisler may want to answer.

##### Checkbox / Label / Select Interaction
- [x] Which element should the label associated with? (select or checkbox)
- [x] ~~If the label is for the select should the checkbox be toggled when clicking the label?~~
- [x] If the label is for the checkbox should focus be given to the select field after clicking the label?

##### Featured Items Widget
In order to not duplicate code as much as possible I have updated the Featured Items Widget's caption field to look similar to the others (since it shares a template) and simply hidden the Secondary caption field.  Check out the attached screenshot at the end of this PR and let me know if that's okay.

- [x] Okay

##### Styling
- [x] Do we want to style the captions differently for the multi-up widget? (currently italicizing both captions)
- [x] Do we want to use different headings in the carousel caption? (currently h3 for primary h4 for secondary)
- [x] Do we want to style the captions differently for the item+text widget? (currently not doing any styling there)

### Multi-Up Admin
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![multi-up-admin](https://f.cloud.github.com/assets/96776/2406868/8c7d40ae-aa7c-11e3-9ae0-b9ae843f84d8.png)

### Multi-Up
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![multi-up](https://f.cloud.github.com/assets/96776/2406870/96fbb4a2-aa7c-11e3-9616-995df842c420.png)

### Item+Text Admin
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![item-text-admin](https://f.cloud.github.com/assets/96776/2406872/af744c92-aa7c-11e3-9598-4a03332127cb.png)

### Item+Text
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![item-text](https://f.cloud.github.com/assets/96776/2406874/b8ad0308-aa7c-11e3-8b73-2e66510f7720.png)

### Carousel
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![carousel](https://f.cloud.github.com/assets/96776/2406876/c3486938-aa7c-11e3-8d42-13e1b1591ad5.png)

### Featured Items Admin
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![featured-items-admin](https://f.cloud.github.com/assets/96776/2406878/cca71e5c-aa7c-11e3-859f-2d6eece11157.png)