Add a few more icons

Adds some additional icons (which may or not be rubbish given my lack of graphic
design skills so please improve if desired).

Icons are added for NT (another NTriples icon) and for RDF/XML

ReadMe is updated to include these
Make some of the new icons a little bigger

Increases the size of some of the new icons to be a little less tiny,
updates the README and downloads to include these
Merging
Merge https://github.com/k00ni/semicon

Conflicts:
	README.md
	downloads/semicon.tar.gz
	downloads/semicon.zip
	png/all/32/rdf.png
	png/terms/32/rdf.png
Update ReadMe to fix conflicts
Merge https://github.com/k00ni/semicon

Conflicts:
	downloads/semicon.tar.gz
	downloads/semicon.zip
Merge https://github.com/k00ni/semicon

This commit merges the latest changes from the upstream master and regenerates the downloads using the merged files from the upstream master

Conflicts:
	README.md
	downloads/semicon.tar.gz
	downloads/semicon.zip
Merge pull request #2 from rvesse/master

New icons for n-triple and rdf/xml