Initial support for importing files from other apps

PNG, JPEG, SVG, TIFF, PDF
Can upload a PDF exported from OmniGraffle!
Can upload an SVG exported from TouchDraw!
Can't thumbnail PDFs or SVGs yet.
Add initial file icon support for SVG & PDF

PDF can be read by CoreGraphics but doesn't go conveniently into a UIImage, needs work.
SVG can be read by WebKit but doesn't go conveniently into a UIImage, needs work.
Icons are low-res and grabbed from MediaWiki, they should be redrawn at high-res. Or replaced with thumbnails.
Merge branch 'filetypes': add support for sharing files from other apps