Stop times are only required for first/last trip stop

Some GTFS feeds fail without this check, e.g. VTA, SamTrans.
platform_code optional column

Needed for some feeds, such as Caltrain
https://support.google.com/transitpartners/answer/2450962?hl=en
Skip empty CSV rows

BART’s current GTFS feed contains some empty lines. These rows with
zero elements break namedtuple; skip these.
Check empty strings as floats

Caltrain GTFS includes empty strings in the shape_dist_traveled field,
which fails when trying to convert to float.
Explicitly check for empty row.

Tuple._make requires the correct length. Instead of just catching a
TypeError, check if the row is empty.
Added explanatory comment
Whitespace fix
Merge pull request #7 from irees/master

Fixes to import various Bay Area GTFS feeds.