Make script run uninstalled.

If the script is not installed via setup.py, inkscapeslides.py is missing.
Now __init__.py can be run directly without installing it.
Build slides list automatically if no 'content' layer is given.

By default, the script looks for a layer named 'content', in which
the slides and their content is specified. If no such layer exists,
the slides list is now built automatically, assuming one layer is
one slide.
Use argument lists with Popen instead of a shell style command line.

When using a shell command line, quoting becomes an issue, e.g. for
filenames with spaces. Using argument lists instead works around this.
Fix spelling.
Merge pull request #15 from meisterluk/master

Spelling fixes.
Merge branch 'master' into autocontents