Refactoring and bug fixes in list styles

The purpose of this PR initially was only to fix some bugs I had encountered in relation to multi level lists. However after spending some time in the area I discovered we had some code that did list styling in OdfCanvas.js that was either dead code or duplicated code from Styles2CSS.js that was added there to enable continued lists. Interestingly the continued list functionality has regressed and does not work in master at this time.

So in this PR I propose an initial refactoring of the list styles code out of Styles2CSS into its own class with a view to remove all the list styles code from OdfCanvas in a future PR. I have also added my minor bug fixes into the new refactored class.

Benefits of this approach are:
* All list style logic is kept together instead of being split across Styles2CSS and OdfCanvas and improves the maintainability of both classes
* Can remove duplicate list style code from OdfCanvas and implement the continued list feature and other unimplemented list styling in the new refactored class without requiring any dependency on Styles2CSS.

I have uploaded a document here containing a multi level list that can be used to verify my fixes: https://drive.google.com/file/d/0B3cHKCJPcW1jc0ZiNHQ4X2VITGM/edit?usp=sharing

Should we start uploading documents such as these to the repository or somewhere else?