Splitting channel metadata into Channel and Manifest files

[jmesnil]

Splitting the Channel definition into Channel and Manifest files.

Channel contains static information - repositories, vendor, etc while Manifest contains streams definition

[jmesnil]

@spyrkob can we step back from #109 and define here what we want to achieve.

In particular, we should define:

• the spec for the 2 schema (manifest and channels)
• how they relate to each other (in particular the requires field)
• the actual definition of the maven repositories and making sure that they can work with different environments (offline installation, proxies, etc.)

[spyrkob]

@jmesnil what's the best way to collaborate on the spec? put it down in a comment here, create a google doc?

[spyrkob]

@jmesnil I created #123 with the updated spec and schema.

I added all the changes to the channels definition we're looking at (manifest, blocklist and default resolution strategy).