Fix requires (avoid require_relative)

require_relative should be avoided inside gem
Move all lib classes/modules under Asciidoctor::LaTeX namespace
Rename lib/asciidoctor-latex to lib/asciidoctor/latex

To follow best-practices for rubygems
http://guides.rubygems.org/name-your-gem/.
Add stem:latexmath to default options in bin/asciidoctor-latex
Fix instructions in readmes
Merge pull request #3 from jirutka/patch-4

Fix requires and directory/module layout