[Decision Trees] Initial dataset and runner scaffolding for census-income dataset.

Summary: Will be extended with training capabilities.

Test Plan:
```
tulloch at tullochmbp15 in ~/Code/go/src/github.com/ajtulloch/decisiontrees/census-income (census-income-tet)
∴  census-income
2013/09/10 01:42:27 numWeakLearners:100 splittingConstraints:<maximumLevels:3 > lossFunctionConfig:<lossFunction:LOGIT >
```

Reviewers:

CC: