Added more resources and organised in categories

Hey @benmarks,

as we talked on Reddit, i took a step and combine your resources with [mine](https://gist.github.com/kbariotis/c8723cc25f5761160c1a). This is what i ended up with.

Some notices that you may make
* I removed the presentation. There are a lot of Magento presentation/talks out there. My thinking is that i should add them all or nothing.
* The same with People. That's why i left only the Magento's Core Team for communication purposes. With all due the respect to others. Should we add more core people there?
* I removed non active resources, with say last blog post being a year ago or with two-three posts a year.

I hope you like it.