Update free-programming-books.md

Added Adobe Flex2 programming book
Merge pull request #141 from teu/master

Update free-programming-books.md
Adding Octave
Merge pull request #142 from nerandell/octave

Adding Octave
Put Flask under Python
Merge pull request #144 from nims11/patch-1

Put Flask under Python
Add Problem Solving with Algorithms and Data Struc

Add Problem Solving with Algorithms and Data Structures. Since it is
written in python, I have added it twice.

This was suggested by Fabian Dablander on the Python community on
Facebook.
Added the Language Agnostic subsections to TOC

Found myself scrolling through the Language Agnostic section a lot; added TOC for quicker access.
Merge pull request #145 from JIghtuse/master

more hackershelf books, some refactoring
Merge pull request #146 from gabraganca/Data_struc

Add Problem Solving with Algorithms and Data Struc
Merge pull request #147 from corragon/patch-4

Added the Language Agnostic subsections to TOC