Minor enhancements

* List PDF files by size when optimizing
* Remove unoptimized PDF when cleaning
Modified checkmark commands, added tikz support

* Fixed babel - tikz issue.
* Reformated _yesmark_ and _nomark_ commands to insert pifont characters instead of EPS graphics.
Begining of chapter 2 (WIP)

Added
* Current problem
* Proposed solution
* Technologies used
* Prototype diagram
* Directory diagram
* File access methods