Cherrypick docs

* Added system/events.md at https://github.com/mozilla/addon-sdk/commit/3a66be193c2ff01f955d12ffa4ed17f1502f7f95#doc/module-source/sdk/system/events.md

* Cherry-pick the following commits:
https://github.com/mozilla/addon-sdk/commit/dad06901db0a73c9c162b5afcfdfe11a0d8ba50a
https://github.com/mozilla/addon-sdk/commit/61835812eb8ab8dee7c9bacb02e208e9829942d7
https://github.com/mozilla/addon-sdk/commit/f8381fd23b05868deef7b90de8a607a1a835038e
https://github.com/mozilla/addon-sdk/commit/dae0223b0cf612568a72547df4a31240b62c43df
https://github.com/mozilla/addon-sdk/commit/eacb7971676fdf8d856eb0e7d0494d3998324f73
https://github.com/mozilla/addon-sdk/commit/2deb7671e7b3333d5366488d90412cade201ea47
https://github.com/mozilla/addon-sdk/commit/06c99651d2c78330cc87e090aa9184688bc3d7c3
https://github.com/mozilla/addon-sdk/commit/6d3b0f16f8263511eab30575147e7c6411982955
https://github.com/mozilla/addon-sdk/commit/b323148c9d12ac637a6d70af91ce1481ebd7f9c3