Extreme surgery to prune branches of the Installfest TOC tree

All OSX users who land on the 'install with RVM' steps are now directed
to install Homebrew, configure RVM to use Homebrew, and use Ruby
2.0.0 instead of Ruby 1.9.3. This may cause some trouble on machines
that can't easily install 2.0.0, but we should be able to work it out.

Split "install rvm with ruby" into "install rvm" and "install ruby" and
included in OS-specific pages as partials where required.

"Install Sublime", "Install Textmate", and "Install Homebrew"
are now used only as partials.

"Configure Git" is now a proper "next step" after all the OS-specific content,
rather than a subpage of all of them.

Nested 'step' or 'option' tags now print their option number as
the joined list of all the step numbers (ex: Step 2.2.1 instead of just
showing "step 1").

The confusing 'choice' method (which can be easily implemented as 'step') is now gone.

Introducing a new 'option_half' method for when two quick-to-explain options
can be shown side by side. Currently they don't expand to have the same height
which sucks a little bit.

Shrink the height of the finder image PNG in 'Install Sublime'