Added a "random" module
Updated NineML version number from 0.2 to 0.3 in Python library.
Small changes in Python user_layer module:

* use <link> tag instead of <url> inside definition;
* allow RandomDistribution, string as parameter values;
* serialization of parameters is now in alphabetical order;
* population positions now optional.
Added XMLReader for structure module; XMLReader in dynamics module now supports https:// urls (previously only http:// and ftp://)
Updated user layer example "simple_example.py" to use GitHub for the catalog rather than the Software Center svn repository.