amber.js tries to find itself also by id not just as last <script>

When loaded all scripts asynchronously, the
'last <script> tag' trick is unable to find the right one,
so amber.js does not see its path and possible data-libs.

In case of async loading, giving the <script> well-known id
'amber-path-mapper' allows it to find itself.
update CHANGELOG with async amber.js loading
Commenting 'amber-path-mapper' id in "find myself" part
Merge pull request #771 from herby/alternate-find-amberjs-script

amber.js tries to find itself also by id not just as last <script>