Added code for capturing \label{} in equation

Now it is possible add equations in the labeled form, as in
  \begin{equation}
    \label{eq:label-here}
    E = mc^2
  \end{equation}
The label is stored in a separated variable, and then the corresponding
string is removed from the code for a proper parsing of TeXZilla.

This is the first step to get numbered equations.
Equations are numbered by LaTeX labels

Following the tip fro @r-gaia-cs, and the tutorial in

    http://rypress.com/tutorials/mathml/tables.html

Tables were used for displaying numbered equations. I made the render
show the equation tag instead of a number, as I could not make automatic
numbering work.
Improved equation label extraction

Adding a non-greedy character to the regular expression for extracting
equation labels fixed some problems with brackets inside equations.
Equations are numbered with numbers

Adding a global variable 'neqs' it was possible to label equations
within a single tiddler by numbers instead of showing the eq:label.
Still missing implement equation references.
Equation number displayed on the left of equation
Added numbering for non-labeled equations
Added support for non-numbered equations

As numbered equations are supposed to be typeset by the
\begin{equation}...\end{equation} environment, non-numbered equations
can be typeset using the starred environment
\begin{equation*}...\end{equation*}
Removed artificial eq. number spacing