chore: simplify gitignore

[filipesilva]

Removed some unused ignored folders in .gitignore and fixed the npm-debug.log ignore entry.

Also simplified the *.js ignore rules to not ignore files in the project root, so that a user wouldn't need to add gitignore exceptions when adding more tooling files (e.g. gulpfile.js).

[wardbell]

Needs modification

I think this might be better:

.idea   
typings
node_modules
jspm_packages
npm-debug.*
link-checker-results.txt
app/**/*.js
*.js.map
_test-output
_temp

!**/*e2e-spec.js        

[filipesilva]

@wardbell updated as per your suggestion.