typed color inputs

![image](https://cloud.githubusercontent.com/assets/1767466/4745970/56aadbc8-5a42-11e4-8a77-132006249170.png)

Instead of using a backdrop pattern, put rects in as our little "eyes". This is necessary anyways so that different inputs can be colored differently.

This also shows the ability to restrict connections to particular types (i.e. I can't drag a string into a number slot). Turns out blockly-core already had support for that.

At some point in the future, we may want to build a library of colors so that instead of specifying the hsv, I can do something like getColorForType('string').