Cleanup Build process

[brean]

The build scripts in gown are just copied over from an older pixi version (with some minor modifications). There is a lot going on that is not needed and complicates the build process unnecessarily.
So we should clean up and create some new scripts that only do what is needed. It can be based on the current PIXI-build scripts but should not be just a copy

[brean]

the current pixi-dev branch is not using using gulp anymore, just npm directly. I like to throw everything out that is not needed and clean it up so I think we should also get rid of it.

[FlorianLudwig]

I like to go with webpack as bundler. I think it is pretty much the most common one right now and it supports many interesting features we can make use of like code splitting.

Moving towards ES2015+ is also something I would like to investigate so having the capabilities within the build system should be considered.

[brean]

It's done. See #93

[FlorianLudwig]

Merged.