Initial commit of tex-converter.rb and associated files.
Major changes since the initial commit

- The preamble and clodsing line (\end{document}) are now written
  under the control of tex-cenverter.rb

- The preamble text resides in preamble.tex

- User macros reside in macros.tex

- Tex macros needed for conversion of Asciidoc to LaTeX
  reside in asciidoctor_macros.tex

- The title, author and date are mandatory and are set in the
  Asciidoc file in the usual way.  They are then mapped
  to the required LaTeX constructs, e.g., \author{John Doe}

- There is support for both LaTeX and Asciidoc math syntax

- All of the processing code in the class LaTeXConverter was
  moved to a set of extensions of the classes of the nodes themselves.  Thus
  "convert" now acts as a dispatcher to methods of ther classes.

- Added support for admonitions. These, however, must
  defined as a TeX macro:

   \newcommand{\admonition}[2]{\textbf{#1}: {#2}}

  This is a pattern used throughout.  For example, block constructs
  in Asciidoc are mapped to a LateX macro call, and the corresponding
  macro is defined in asciidoctor_tex_macros.tex

- Converter now looks for the :toc: attiribute.  Inserts \tableofcontents accordingly.

- <<< = page_break now supported (\vfill\eject)

- Nested lists now work.

- Literal blocks now supported.

- Labels (explicit id's) such as [[foobar]], are now supported

- Cross-references <<foobar>> are now supported.  See README for
  comments.

- Implemented block pass-through: ++++ ... ++++

- Added: postprocessing to tex elements to deal with
  unprotected characters such as &, >, and <.  Testeb
  by file sample3.ad.  The strings "&amp;", "&gt;"
  and "&lt;" are mapped to &, >, and <.  Not a great
  solution, but it works.

- tex-converter now handles matrices properly (had to deal with the \\
  in the matrix environment).

- Protected symbols &, >, <, \\ are now processed correctly for stem mode.

- Very basic support for quotes

- Primitive support added for the \newtheorem environments.  See
  sample3.ad for an example.

= Implemented footnote

- Added support for listing blocks
- Moved the tex-converter project into ruby/latex

- Rename: tex-converter --> latex_converter

- Added @mojavelinux's method: initialize backend, opts to latex_converter
use built-in verbose controls

- use built-in verbose controls to control output ($VERBOSE)
- output to stderr instead of stdout
- fix placement of initalize method in converter
fix usage instructions for latex converter
cleanup the README for the LaTeX converter
Merge PR #64 by @jxxcarlson