Implement dynamic toolbox filters.

Added well documented lib/galaxy/tools/filters/examples.py.sample file that outlines the implementations of some possible filters.

Refactored Toolbox.to_dict, moving all filter building functionality out tools/filters/__init.py. Implemented new dynamic filters for tools, labels, and sections. Multiple such filters can be specified by giving module (relative to galaxy.tools.filters) and function names for each as comma seperated list to the respective options tool_filters, tool_label_filters, and tool_section_filters in universe_wsgi.ini. Explicit example lines to add to universe_wsgi.ini can be found in examples.py.sample described above.