Removed learning materials

Closes https://github.com/PistonDevelopers/piston/issues/675
Merge pull request #676 from bvssvni/master

Removed learning materials
Fixed doc description

Closes https://github.com/PistonDevelopers/piston/issues/471
Merge pull request #677 from bvssvni/master

Fixed doc description
Fixed window back-end links

Closes https://github.com/PistonDevelopers/piston/issues/678
Updated README

Closes https://github.com/PistonDevelopers/piston/issues/679
Closes https://github.com/PistonDevelopers/piston/issues/680
Added list of features to README

Closes https://github.com/PistonDevelopers/piston/issues/681
Fix missing_doc warning
Merge pull request #684 from darinmorrison/master

Fix missing_doc warning
Fix typo in readme
Merge pull request #689 from aochagavia/patch-1

Fix typo in readme
EventIterator has been renamed to Events upstream
Merge pull request #690 from eklavya/patch-1

EventIterator has been renamed to Events upstream
Removed `EventSettings`

Closes https://github.com/PistonDevelopers/piston/issues/694
Merge pull request #695 from bvssvni/master

Removed `EventSettings`
Removed AssetStore

Closes https://github.com/PistonDevelopers/piston/issues/704
Merge pull request #705 from bvssvni/master

Removed AssetStore
Added current objects for playground experimenting

Closes https://github.com/PistonDevelopers/piston/issues/706
Merge pull request #707 from bvssvni/master

Added current objects for playground experimenting
Fix Travis CI

Install SDL2 to compile