Add first GTFS accessor: us_trimet

- The zipped feed is ~15MB, which extracts to ~110MB
- In the database it uses ~140MB, where approx. the half is used
for indices, which are used to make the used SQL queries fast
- This is the first official data source and they have some courtesy
text to be shown, this text is in the <credits> tag of the accessor
XML. TODO: Always show this text in the applet, instead of
"data by: XXX"