Additional kwargs for initialization of Tester() and optional HTML checking

This pull request should be save: I added a few optional kwargs: cleanup, checkHtml and executable.  
The html checking can be disabled with:

```
t=regressiontest.Tester(checkHtml=False)
t.run()
```

And on linux, you can now create the (dymola) mos files without error by doing:

```
t=regressiontest.Tester(checkHtml=False, executable='omc', cleanup=False)
t.run()
```

The next step is to make slightly different ``runAll.mos`` files when the executable is 'omc'.