fixes multiple issues (broken links to be deleted)

fixes #738
fixes #727
fixes #725
added nicholas c. zakas understanding ECMAScript 6
Merge pull request #839 from hiteshtr/master

added nicholas c. zakas understanding ECMAScript 6
Merge pull request #838 from borgified/fix_738

fixes multiple issues (broken links to be deleted)
Added books by Peter Kropff
Verbesserungen
Merge pull request #840 from merkuriy/patch-1

[de] - Added books and slight improvements for German list
[pt_BR] - Kanban e Scrum - Fixes #832
[pt-BR] - Fixes #832
[pt_BR] - Fixing Index
added an intro to databases class
Merge pull request #841 from jay754/master

added an intro to databases class
added think complexity pdf
Add Angular.js and Postgres books.
Merge pull request #842 from levibostian/add-more-succinctly-books

Add Angular.js and Postgres books.
added mit's database course
typo
Merge pull request #843 from jay754/master

Added MIT database system
Alphabetize Oberon, Objective-C, Postgres
Merge pull request #844 from jwworth/task/alphabetize

Alphabetize Oberon, Objective-C, Postgres