Configure Mathematical to only work as an itex -> SVG converter

In proper Unix fashion, every piece in this math setup should do one thing and one thing only. 

Mathematical will now take just itex-style strings (`$..$` or `$$..$$`) and turn them into SVGs. You can pass the option `:base64 => true` to return a base64-encoded string instead of the usual `<svg>` XML:

```ruby
MathToItex(string).convert do |eq, type|
  svg_content = Mathematical::Render.new(:base64 => true).render(eq)

  # create image tags of math with base64-encoded SVGs
  %|<img class="#{type.to_s}-math" data-math-type="#{type.to_s}-math" src="#{svg_content}"/>|
end
```

/cc @bkeepers