Introduce commands to allow creating sequence diagrams within tikzpicture environment

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If you use externalization library, it looks for \end{tikzpicture}, which is 
not explicitly present in case of sequencediagram environment. That's why it 
would be useful to be able to create sequence diagrams within tikzpicture 
environment directly.

My proposal for names of the new commands would be: \sdinit, \sdbegin and 
\sdend.

sequencediagram would be redefined then as follows:

% the environment of sequence diagram
\newenvironment{sequencediagram}
{\sdinit\begin{tikzpicture}\sdbegin}
{\sdend\end{tikzpicture}}

You may see also:
http://tex.stackexchange.com/q/98525/5264

Original issue reported on code.google.com by [email protected] on 17 Feb 2013 at 11:59