fix unit test `cannot register twice`
add license to mixin-manager.tests.js
remove direct acces to globals of mixin-manager.tests.js
improve "confusing id registering" unit test
renaming some tests
short test improvement
mixin-manager.js jslint + reduce number of line
add feature to mixin-manager to parse strings
use local json parser instead of JSON.parse
add some tests mixin manager parse from string