Embedded functionality and docs

[diafygi]

The design tool should be able to be embedded in other websites to allow others to offer their own design widget for their users free of charge (the more masks the better!). Thus, we need to enable embedding functionality and write developer documentation for customizing and interacting with the embedded iframe.

Here's a list of some sub-tasks for this issue:

• Docs - overview and quickstart
• Docs - customizing what is shown
• Docs - postMessage API docs
• Detect when the tool is embedded and hide header/footer/etc. so just the design tool is shown.
• Add support for url parameters for interface customization (e.g. only show the 3d preview).
• Add support for postMessage to set the state of the design tool (e.g. load a given image).
• Disable support for sharable url generation when embedded (up to their website to make their own short urls).