added wheelchair_boarding attribute to stops
added wheelchair_accessible attribute to trips
transfers isn't a required attr (can be blank),
add fare_ column prefix
tests for fare attributes
add missing unit tests
Merge branch 'master' of github.com:s2t2/gtfs into wheelchairs
Merge pull request #10 from s2t2/master

update with wheelchair-related attributes
#9 Add ability to iterate through instances of each Model without loading that Model's CSV file into memory.
Merge pull request #11 from mikepfirrmann/master

Resolve #9
Add doc for parsing method w/ each