Adding Python interactive tutorial
Re-added Category Theory for Computing Science with udpated link.
Merge pull request #920 from jeffdc/master

Re-added Category Theory for Computing Science with updated link.
PDF about language php
Add EmberFlare and Dockyard tutorial
Merge pull request #922 from h0lyalg0rithm/patch-1

Add EmberFlare and Dockyard tutorial
Merge pull request #921 from gutierri/master

PDF about language php
Merge pull request #924 from esparta/master

[en] some clean on free-programming-book.md
A Chinese translation of 《UNIX TOOLBOX》
Merge pull request #925 from Cupen/patch-1

A Chinese translation of 《UNIX TOOLBOX》
Merge branch 'master' of github.com:vhf/free-programming-books
[en] Sorting Software Architecture section
[en] Sorting Clojure section
[en] Sorting Emacs section
[en] Sorting F# section