ACE2 refactoring

[fourplusone]

This pull request introduces 2 new modules

• ChangesetUtils.js, basically code from ace2_inner.js moved into it's own module
• AttributeManager.js, a class which manages the attributes of the text (unfinished, since some code is still in ace2_inner.js)

+ several minor (code style) improvements

[yadutaf]

This is a great start. We need that in. Can you add a documentation block above your functions and at the top of the file ? Later we will need to refqctor the equivalent for range styles.

[fourplusone]

Just push'd some more docs + moving performDocumentApplyAttributesToRange to AttributeManager.setAttributesOnRange