Add build-es and build-lib gulp tasks

[hourliert]

Fix #138

We now build a unbundled version of the app under the folder lib.
An ES2015 version understandable by recent bundlers like rollup and webpack2 is also available under the folder es.

I have tested this new setup with webpack1 and webpack2. It works great! Webpack is finally stop yelling about:

WARNING in ./~/inversify/dist/inversify.js
Critical dependencies:
7:482-489 This seems to be a pre-built javascript file. Though this is possible, it's not recommended. Try to require the original source to get better results.
 @ ./~/inversify/dist/inversify.js 7:482-489

Like for the bundled version. I have included the banner (containing copyright, license and version) in each file of the folders lib and es.

Tell me if something is missing.

[coveralls]

Coverage remained the same at 100.0% when pulling 470739e on hourliert:lib-es-build into 8d43665 on inversify:master.

[remojansen]

@hourliert give it a few minutes and run the build again the coveralls servers fails some times :(

[coveralls]

Coverage remained the same at 100.0% when pulling c861b91 on hourliert:lib-es-build into 8d43665 on inversify:master.

[remojansen]

Nice one 👍 thanks for your help!

[remojansen]

@hourliert If you are wondering when will this be release... we have a roadmap. I want to merge issues #142 & #136 and then I will release 2.0.0-alpha.7. After that we should be really close to a beta release.

[hourliert]

@remojansen Yeah! Can't wait to see alpha.7 out.

And BTW, this library is awesome. Great job!

[remojansen]

Thanks!