Update README

Add npm ERR section
Update README
Add gif and webp in "copy" task

As gif is not supported by imagemin, it needs to be manually copied:
https://github.com/gruntjs/grunt-contrib-imagemin/issues/29#issuecomment
-15619821
Remove SCSS references in README

I don't want to use grunt-contrib-sass (needs ruby), and grunt-sass
(uses nodejs) is not mature enough. Planned for later.
0.2.5
Merge branch 'master' into api-auto-discover

Conflicts:
	package.json