Fixes #118 - Allow other elements inside iron-input

[AlexHenkel]

This fixes #118 by explicitly selecting <input> child element, instead of just assuming it will be the first child element.

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[AlexHenkel]

I signed it!

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CLAs look good, thanks!

[notwaldorf]

As described in the issue, iron-input is a wrapper for just input, and shouldn't really contain anything else inside of it