Added a post on the injectivity of stable mappings

The draft of this post was actually written on the `source' branch.
Draft for my additional list of Git commands
Merge branch 'source' into wpcom-importer
Added a new post on content 0 sets

Corrected some mistakes in two recent posts about math.
Updated my recent post about Git

I corrected a conceptual mistake.
Merge branch 'source' into wpcom-importer
Updated my newest post
Merge branch 'source' into wpcom-importer