Compile the examples at once

This PR changes the `Makefile` to make the `check` target compile the examples all at once, instead of individually compile each example. To goal of this PR is really to speed up the build process.

This also means that we no longer send compiled versions of the examples on github.io/gh-pages. So the examples on github.io are not compiled and run against `ngeo.js`.

Also, as indicated in the README, the examples can still be individually compiled. For example:

```shell
$ make .build/examples/interactiontoggle.min.js
```

Please review.

fixes #30