add support for <script> in <scxml>

[sstiller]

A better version of https://github.com/jp-embedded/scxmlcc/pull/78. #include, type declarations, etc. can now be added. Example:

<scxml initial="hello" version="0.9" xmlns="http://www.w3.org/2005/07/scxml">
  <script>#include &quot;foo.h&quot;</script>
  <script>void hello()
{
  std::cout &lt;&lt; &quot;Hello&quot; &lt;&lt; std::endl;
}</script>
  <state id="hello"/>
</scxml>

results in

...
// user initialisations
#include "foo.h"
void hello()
{
  std::cout << "Hello" << std::endl;
}
...

Closes #63

[jp-embedded]

That was fast - great :-)