fix: replace code related to `gitignore`
fix: replace everything for `platforms/.gitkeep`
feature: create automatically `platforms\.gitkeep` and default lines in `.gitignore`

At first, I was generating these files on `grunt phonegap:check` but it
felt wrong and was easy to miss.
Merge branch 'master' of github.com:dsimard/grunt-angular-phonegap into development