[WIP] Post Filter Menu

closes #2640
- Adds post filter
- Adds gh-popup to show filter menu
- Implements more filters on posts.api

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This PR implements the post filter menu. A live example can be seen in the screencast below (styles would need adjusting - they are inherited from `class='content-list'`). There's a bug with URL handling so that when you refresh the page with a query param active, it will append a trailing slash and screw up a ton of stuff. Not sure if it's us doing the trailing slash or queryParams not handling the our trailing slash correctly. I haven't yet looked into proper pagination, but it should be fairly simple to resolve.

![post-menu](https://cloud.githubusercontent.com/assets/176576/4173659/1f69bf32-3560-11e4-84a2-00ffba881254.gif)

I realise the issue is in the 0.6 milestone - no hard feelings if this isn't going to get merged, this can easily serve as a PoC. Working on this helped me get a better understanding in the whole Ember magic.

Won't this break infinite scrolling in the posts list?