Added option 'cleanupdefs' (default: false); When set to true no cleanup is done on elements inside the <defs> element.

Changed option 'cleanup'; it now accepts an array of attributes. For backwards compatibility cleanup === true equals ['style']. The attributes in the array are removed from all elements.
Empty g elements are removed since they have no effect in a document.
Minor speed improvement: only compute uniqueId (at most) once.
Merge pull request #42 from Frank3K/master

Remove empty g elements