REXML is dead. Long live Nokogiri.

Modify Maruku to use Nokogiri instead of REXML.
Produces a 3-fold speedup in the #to_html method.

vendor/plugins/maruku/spec/block_docs/math/notmath.md

@@ -0,0 +1,25 @@
+
+*** Parameters: ***
+require 'maruku/ext/math'; {:math_enabled => false}
+*** Markdown input: ***
+
+This is not $math$.
+
+\[ \alpha \]
+
+*** Output of inspect ***
+md_el(:document,[md_par(["This is not $math$."]), md_par(["[ \\alpha ]"])],{},[])
+*** Output of to_html ***
+<p>This is not $math$.</p>
+
+<p>[ \alpha ]</p>
+*** Output of to_latex ***
+This is not \$math\$.
+
+[ $\backslash$alpha ]
+*** Output of to_md ***
+This is not $math$.
+
+[ \alpha ]
+*** Output of to_s ***
+This is not $math$.[ \alpha ]