Arduino build doesn't support subdirectories

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Since the beginning, we've wished we had subdirectories for organization.

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As it turns out, recent versions of Arduino (~1.6) do support this. Here's where I found this information.

• https://github.com/arduino/Arduino/issues/5176
• https://github.com/arduino/Arduino/issues/1004

And ~1.6.10 restricts this behavior to src/ to prevent unintended consequences.

Using this knowledge I've created #100.

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Waiting on #62.