add yarn integration

[gruppjo]

as part of #468 (yeoman 1.0 integration).

Don't forget to update docs!

[gruppjo]

related (check in deps) #410

[gruppjo]

Apparently, there's is some issues with installing protractor through yarn

[gruppjo]

@lordgreg: yarn should run pre and post install script as of yarn 0.16 https://github.com/yarnpkg/yarn/releases/tag/v0.16.0 // yarnpkg/yarn#800.

However, I tried it with 0.19.1 and gulp protractor is still not running out of the box. Maybe there's another problem.

[lordgreg]

@gruppjo the only issue I've found is using Windows and Yarn here.

Other than that, I will start asking in Protractor / Yarn github pages about the issue.

[gruppjo]

• discussion over at yeoman/generator for .yarnInstall() - Feature: New installation method for Yarn yeoman/generator#964
• yeoman update guide - http://yeoman.io/blog/hello-generator-1.0.html
• yeoman documentation - http://yeoman.io/generator/actions_install.html
• jhipster also going for yarn - [Feature] Official support for Yarn jhipster/generator-jhipster#4317

[gruppjo]

@lordgreg, I created a separate issue for the yarn/gulp-protractor issue (which I linked above).

I'm closing this one since I finished integrating yarn with the next commit.