Make align-items: stretch the default

I took #13 and made it work correctly.

In the browser tests, unfortunately the root node parent is body which has a width of 300, so using stretch will make all the test fails because their width are going to be 300. In the C/Java tests, the parent is 0. What I did was to hardcode the root node to be align-self: 'flex-start' by default to make it work correctly on both environments.

Also, instead of changing the values for the tests that were failing, I added align-self: 'flex-start', this ensure that we test what we wanted to.

I also updated the README and tested it in our integration, all good