add "The Python GTK+ 3 Tutorial"
Merge pull request #1064 from taurenchieftain/master

add "The Python GTK+ 3 Tutorial"
Fix formatting
Merge pull request #1066 from uschek/master

Fix formatting
Update CONTRIBUTING.md
Isabelle/HOL - fix anchor link

Anchor link for Isabelle/HOL was not working when viewing the Github parsed file.
Update free-programming-books.md

No longer published
Merge pull request #1070 from olso/patch-1

Update free-programming-books.md
Merge pull request #1067 from codrineugeniu/master

Isabelle/HOL - fix anchor link
Removed duplicate link to Dashing d3.js.
Added a new book on Titan Graph Database

Added a new book on Titan Graph Database
Merge pull request #1072 from ridrees/patch-1

Added a new book on Titan Graph Database
Merge pull request #1071 from chrisspruck/quickedits

Removed duplicate link to Dashing d3.js.