Support unicode via pd.read_csv or backports.csv

[fritzo]

Python2's builtin csv library doesn't handle unicode. This is currently causing discrepancies between python2 and python3 values.csv files. Possible solutions include switching to the pandas.read_csv function or the backports.csv library.

[fritzo]

4ceee25 added a treecat.format.read_csv based on pd.read_csv.