Clean up after myself.

Revert unwanted changes from merged Nokogiri branch.
Rollback removal of s5_theme property

We NEED this.
Fix S5 output
Switch to a DocumentFragment for to_html
Use a single cached XML::Document instead of creating them all over
Introduce shorthand for creating XML elements and text nodes
Clean up lib/maruku/output/to_html.rbx
Basic indentation and whitespace for lib/maruku/output/to_latex.rb
Clean up lib/maruku/output/to_latex.rb
 Fix errors in IAL parsing

These didn't used to cause an error.

I think it's OK to hit an EOF in these
cases. So we don't call that an error.
Strip leading/trailing whitespace
Fixes
Rename to_latex_entities.rb to entity_table.rb
Make latex entities omit {} whenever possible
Address a couple Ruby warnings
Fold to_latex_strings.rb into to_latex.rb
Convert the failing span test to "pending"
Clean up lib/maruku/output/to_markdown.rb and emit a deprecation warning when #to_md is used
Clean up lib/maruku/output/to_s.rb and emit a deprecation warning when #to_s is used
Clean up lib/maruku/output/s5/to_s5.rb