[es] Added two C# books
Merge pull request #747 from esparta/master

[es] Added two C# books
[es] Added Fundamentos de Sistemas Operativos
Merge pull request #748 from esparta/master

[es] Added Operating System Book
Added a new PostgreSQL book
Added PostgreSQL to the index
Merge pull request #750 from therufa/master

Added new PostgreSQL book
[fa_IR] Ubuntu -> Persian Ubuntu Book
Merge pull request #751 from 0x55D/persian-ubuntubook

[fa_IR] Ubuntu -> Persian Ubuntu Book
[es] Removing duplicated Pro-git book
[es] Adding Programación concurrente y en tiempo real
Merge pull request #752 from esparta/master

[es] Removing duplicates & added book about concurrent programming