Remove failed challenge of 2013-06: Read GEB

Remove the 2013-06 directory as well as its entry from the project
README. This project failed due to the following reasons:

  1. The content was too heavy to be read in 25 days. Some chapters
     required more time than I had anticipated earlier.
  2. Personal issues due to which I missed more than 5 days of reading.
     Since I was already going slow, this made recovery infeasible.

The book is divided into two parts: Part I and Part II. It would make
more sense to read both parts separately as two 25 day challenges in
future.
Add challenge for July 2013: Write QuickQWERTY 1.0