fix: replace `platforms` by `platforms/**/` in .gitignore

Closes #25
fix: make sure that the `platforms/` directory exist

PhoneGap seems to have a lot of problems when trying to add a platform
via `cordova platform add android` if the `platforms` directory doesn't exist.

`grunt phonegap:check` now creates the following file `platforms/.gitkeep`

Closes #24
improve platform message
Merge branch 'development'