Download coursier on the first run

[laughedelic]

• atom-language client kindly provides some utils for this: https://github.com/atom/atom-languageclient/blob/master/lib/download-file.js
• examples:
  • ide-java (with status bar updating)
  • ide-php (with busy-signal updating)

It could be also useful to check if user has coursier already installed and just use it.

[laughedelic]

Instead of downloading solved this by just adding coursier jar to the distributed package. So user downloads it once on install and that's it.