feat(karma): wiredep karma configuration

Update Gruntfile and generated karma configuration for wiredep to wire bower dependencies and devDependencies into karma configuration.

This also replace manual wiredep call of installDependencies callback by an invocation of grunt wiredep using spawnCommand, avoiding duplicated code.

Fixes #856
But i don't know where to put this code elsewhere. `karma.conf.js` is generated by `karma:app` invocation. Maybe we need modify [generator-karma](https://github.com/yeoman/generator-karma) to add a `wiredep` option ?