Add gulp build tasks for examples

[jelbourn]

We should build the examples as a component library similar to how we build the material library itself and include the build output in the bundle that's published to material2-docs-content.

@devversion would you be able to tackle this?

[devversion]

@jelbourn Sure. I just hope we can make the changes without interfering with #3443.

[jelbourn]

Coming back to this: we should use the same code for building both lib/ and examples/, where the only input needed should be the root directory.

[devversion]

Yeah that's what I planned to do. Let's take a look at this next week.

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