Add ID support to paper.
Modified gradient creation and management mechanism to re-use gradient elements for same gradient attributes.

The change is primarily associated around `addGradientFill` method of SVG renderer. The method now re-uses a previously created gradient element that has same attributes. Change was also made to `element.remove` function to ensure element definitions are removed in a managed fashion. For ease of management, a new function called `updateGradientRefernce` has been added.
Added test cases to unit test / functional test gradient element re-use management
Added performance test spec for gradient fill of element.
Updated minified/concat files.
Merge pull request #11 from fusioncharts/feature/single-def-per-gradient

Updated gradient element handling to create only single gradient element per unique fill