Split mittsu gem into mittsu-core and mittsu-renderer-opengl

[Floppy]

This is an early PR for my attempt to resolve #115 by splitting mittsu into two gems. I think it can all be done in the same repo, and I've made it so that mittsu gem itself simply depends on the two component parts.

I've kept a single version string, and made them depend on that exact version, to avoid any differences causing problems. Can always be split later on.

Resolves #115

[danini-the-panini]

Would we do this in subfolders, like Rails does?

[danini-the-panini]

Also is this a patch or minor version bump, since it doesn't really change the interface

[Floppy]

Yeah, subfolders is probably a good idea. And you right, technically by semver it would be a patch version, as absolutely nothing would change! Hah.

[Floppy]

Sorry, I never got much further on this. Hopefully I can come back to it soon.

[Floppy]

@danini-the-panini I think this is ready for you to take a look at. I've split it into separate folders rails-style, and there is a top-level mittsu gem for compatibility with existing installations, who shouldn't notice a difference.

gem build appears to work for all 3 gemspecs, though how this would change your release process I leave up to you.

I'm having trouble running the examples (because of #130), so I can't actually see anything, but the tests seem good; although, I can't tell if coverage upload is still working.

[Floppy]

OK, after rebasing #131, the examples now run, which is great!

[danini-the-panini]

This is going to cause some nonsense now, since according to gem naming conventions, mittsu-core implicitely requires mittsu/core, which is not desirable in this case since it is only a subset of Mittsu's actual core functionality.

Considering Mittsu is pre-1.0, we can still reasonably introduce breaking changes in a minor version bump, and I think in this case it is warranted.

I'm going to split out the opengl renderer into its own gem and keep mittsu as a non-renderer 3d library. I think it makes sense to make it an extensible server-side analog to Three.js, with desktop-side rendering as a separate feature.

As for the opengl renderer, I'm going to move it to its own repo entirely.

[Floppy]

Makes sense to me!

[Floppy]

I think that complete separation is a cleaner approach than what I've tried to do here with keeping compatibility and repo structure, so I'm 100% on board. Feel free to take anything useful from what I've done here, but also just throw it away if it's easier :)