add watch-detector

[stefanpenner]

• fixes add utilize to "autodetect" best watcher. #121
• tests
• bump node engine support: bump engine support to node version that are supported. #123

[stefanpenner]

@xcambar should we add support here as well?

[xcambar]

Do you mean we should add support for watchexec in watch-detector? I did it locally to make it work as easily as possible in ember-cli, I'll create a PR tomorrow.

[stefanpenner]

@xcambar do you notice improvements using watchexec over watchman, if so what? I haven't had a chance to use it yet...

[xcambar]

Nothing measured.
Impressions though, on mid-size apps,are good.

Overall, I prefer the model of watchexex over watchman (single process...)

I'll try to find some time to come up with numbers.

[stefanpenner]

@xcambar cool :)