Merge pull request #39 from Frank3K/style-changes-1

Style changes (code and comments)
Changed the way ID are collected. All referenced IDs are collected and mapped to uniqued IDs. Unreferenced IDs are removed in order to save space.
Add missing file.
Minor speed improvement: only compute uniqueId (at most) once.
Merge branch 'id-references' of github.com:Frank3K/grunt-svgstore into id-references