[Fix] Inline latex formula alignment

Fixed an issue of latex inline formulas not aligning correctly with
the text. The fix is a ruby translation of the Perl code described in
http://tex.stackexchange.com/questions/44486/pixel-perfect-vertical-alignment-of-image-rendered-tex-snippets.
This code calculates the alignment of the generated image depending on
the image size and the size of the white space between the formula and
the margins of the image.

The alignment now is nearly perfect, however the new rendering comes
with a performance impact because of the increased number of operations.
To reduce this impact, the generated images and calculated values for
the alignment are now cached in `~/.cache/gollum`. The caching is done
per formula and not per page, thus avoiding caching the same formula
twice.

The Readme and tests have been modified accordingly with the new tool
requirements and expected markup.

lib/gollum/markup.rb

@@ -146,7 +146,22 @@ module Gollum
     def process_tex(data)
       @texmap.each do |id, spec|
         type, tex = *spec
-        out = %{<img src="#{::File.join(@wiki.base_path, '_tex.png')}?type=#{type}&data=#{Base64.encode64(tex).chomp}" alt="#{CGI.escapeHTML(tex)}">}
+
+        # Obtain the formula with parameters
+        out = nil
+        begin 
+          width, height, align, base64 = Gollum::Tex.render_formula(tex, true)
+
+          # TODO: Should we load the binary inside the html?
+          #out = %{<img width="#{width}" height="#{height}" style="vertical-align: #{align}px;" src="data:image/png;base64,\n#{base64}" alt="#{CGI.escapeHTML(tex)}" />}
+
+          # Use the alignment values from the formula rendering but still use the call to '_tex.png'. Although it will call render_formula()
+          # again, it will use the already cached formula and it might have some advantages from the point of view of browser caching (really not sure here).
+          out = %{<img width="#{width}" height="#{height}" style="vertical-align: #{align}px;" src="#{::File.join(@wiki.base_path, '_tex.png')}?type=#{type}&data=#{Base64.encode64(tex).chomp}" alt="#{CGI.escapeHTML(tex)}" />}
+        rescue # In case of error
+          out = CGI.escapeHTML(tex)
+        end
+        
         data.gsub!(id, out)
       end
       data