Add external links

Note: Blender add-on is not part of the 'Tools' folder, but I think it fits well in the Tools section.
Merge remote-tracking branch 'remotes/mike3d/patch-4'
Added ParticleEditor2D to Related Projects. Removed BlenderExporter section from Tools page, instead mention it in AssetImporter's section as an alternative.
For Travis CI - switch CI build to use OS X build environment.