The project templates for Visual Studio 2012 contain a reference package with javascript, localization files and styles.

What I am wondering is how to build a custom reference package with my code, locales and styles. There is absolutely no information about this on the web, I tried referring to MSDN, but it did not give me any clue as to how I can do this. Please share any useful information that you may have about this.
It turns out Microsoft do have documentation on Extension SDKs.
Update:
I actually managed to get the reference/framework package. It all works really well except when opening Expression Blend with my package referenced. Notice the reference is there and I can actually see all components and their description but I’m still getting an error in the design surface. Here is what the error looks like (I know, it’s very descriptive):

Update:
When trying to deploy an app referencing the framework package I have created I being given an error saying that it cannot find my framework but instead finds some framework with trailing symbols that I did not put there while packaging it. It appends the processor architecture as well as the package family name trailing symbols. Here’s a screenshot:

The item you are looking at is a “Framework Package”. While it is possible for you to generate a package that looks like a framework package (look at the appxmanifest.xml for the WinJS package in
\Program Files\WindowsApps), you cannot get that package ingested into the store.This is something that is not supported in Win8 — you’ll need to include the files you need in each package you build.