PNG size optimisations, fix icon paths in firefox app (and use our logo)

- optipng all the images!
- we're not using glyphicons-halflings.png (it's coming from a CDN)
- replace WeeChat logo with Glowing Bear logo in Firefox app & fix paths
Crunch the svg as  well
Merge pull request #314 from glowing-bear/icons

PNG size optimisations, fix icon paths in firefox app (and use our logo)