Emscripten rework (#286)

* partial implementation for emscripten

this pull request contain a partial but working implementation of emscripten backend

some implementations may be controversial.

here some implementation detail:

* cursor state:
  * on grab: emscripten request pointer lock deferred and also set a callback when pointer lock change
             the callback request pointer lock deferred.
  * on hide: `emscripten_hide_mouse` exist but not `emscripten_show_mouse`
             a pull request has been open on october 2016 but never been merged
             so I copied the javascript function and put it in emscripten_asm_const function

* fullscreen: if fullscreen is requested then it request fullscreen deferred and set a callback on fullscreen change
              the callback request fullscreen deferred

* run forever: this method use emscripten main loop to run an infinite loop

* keyboard callback doesn't consume the event. I think it is more apopriate as in desktop environment it is the same, is it ?

* emscripten dir is added in example and contains html pages

Some things that are not implemented:

* lots of events

* min and max dimension can be implemented with a callback that listen to size change and resize if dimension out of bound

* title may be implemented using javascript to change document.title

* Use std::os::raw in the emscripten bindings

* Fix emscripten code

* Update code

* Add CI

* Remove the emscripten-specific examples

* Add some information to the README

README.md

@@ -41,3 +41,15 @@ fn main() {
     });
 }
 ```
+
+### Platform-specific usage
+
+#### Emscripten and WebAssembly
+
+Building a binary will yield a `.js` file. In order to use it in an HTML file, you need to:
+
+- Put a `<canvas id="my_id"></canvas>` element somewhere. A canvas corresponds to a winit "window".
+- Write a Javascript code that creates a global variable named `Module`. Set `Module.canvas` to
+  the ID of the `<canvas>` element (in the example this would be `"my_id"`). More information
+  [here](https://kripken.github.io/emscripten-site/docs/api_reference/module.html).
+- Make sure that you insert the `.js` file generated by Rust after the `Module` variable is created.