add sample rows method
updated linked tables in transition model to improve performance
tweak and debug sample_rows method
remove debugging from sampling.py, update transition.py to use sample_rows and tusaccounting column
debug acounting column transtion model updates
add new line to end of sampling.py
transition and sampling pep8 fixes, handle linked tables with no added rows
a few more pep8 fixes
add unit tests for sampling, add_rows and remove_rows in transition
add and modify tests for transtion, sampling; pep8 fixes
Merge branch 'master' of https://github.com/synthicity/urbansim
move index comparison (order agnostic) test from test_transtion to utils/testing; also add tests for index comparison
Merge pull request #102 from bridwell/master

transition model updates

Faster linked table updates via pandas.merge and optional accounting of transition table rows.