[pdf creation fix] Added etoolbox installation into README

Without ``etoolbox`` i got this error on my ubuntu 12.10:

````
➥ sudo apt-get install pandoc
➥ sudo apt-get install texlive-latex-base
➥ sudo apt-get install texlive-fonts-recommended
➥ sudo apt-get install cm-super
➥ make pdf 
awk 'FNR==1{print ""}{print}' chapters/*.md > 'backbone-fundamentals'.md
# You need `pdflatex`
# OS X: http://www.tug.org/mactex/
# Then find its path: find /usr/ -name "pdflatex"
# Then symlink it: ln -s /path/to/pdflatex /usr/local/bin
pandoc -s 'backbone-fundamentals'.md -o 'backbone-fundamentals'.pdf \
		--title-prefix 'Developing Backbone.js Applications' \
		--normalize \
		--smart \
		--toc
pandoc: Error producing PDF from TeX source.
! LaTeX Error: File `etoolbox.sty' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)

Enter file name: 
! Emergency stop.
<read *> 
         
l.28 \def

make: *** [pdf] Ошибка 43
````