Method for accessing all values of set with Element.data

I propose a method Element.getData that returns all data set with Element.data. Currently the only way to get data from an element is by knowing the keys.

I require this functionality for [Raphael.JSON](https://github.com/ElbertF/Raphael.JSON) which can be used to save the state of a paper.

**Edit:** I just noticed my editor deleted trailing whitespace all over the place. The actual diff is on this line:

https://github.com/DmitryBaranovskiy/raphael/pull/510/files#L0R2101

I can submit another pull request if the suggestion is accepted.