Improvements to Pep8analysis Web version

* Support C++ in the emscripten platform
* The emscripten platoform support for the `--release` option reduces the file size of 50%
* pep8analysis error messages are now friendlier
* Do not spend as much time trying to find a fixed point (which is much faster!)
* Use Google Analytics to collect data and events
* Report fatal crash of the tool to the user
* Notice the user when the tool is working
* Does not lauch the full main for each analysis, only call a single Nit method
* Launch computation async using web workers when supported

The stack size of web workers are limited in Google Chrome under Linux, you are better off using Firefox to test big programs. Maybe they should be deactivated by default under this OS and browser.