Added free eBooks that I am aware of.

I had a blog post here:

http://www.gep13.co.uk/blog/free-ebooks/

which listed some free eBooks that I was aware of.  I have taken the programming related ones, and added them to this list.
Corrected links

Corrected the internal link to the Windows 8 section.
Corrected the external link to Windows Phone Blue Book
Merge pull request #283 from gep13/master

Added free eBooks that I am aware of.