[Decision Trees] Added .gitignore for test files

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D21
[Decision Trees] Remove logging in FastTreeEvaluator::evaluate method

Summary:
Speeds up evaluation somewhat:

```
∴ go test -benchmem -bench=Tree -cpuprofile /tmp/cpu.out -blockprofile /tmp/block.out -memprofile /tmp/mem.out -num_features 1000
PASS
BenchmarkFastTreeEvaluation     20000000                81.2 ns/op             0 B/op          0 allocs/op
BenchmarkNaiveTreeEvaluation    20000000                91.1 ns/op             0 B/op          0 allocs/op
ok      github.com/ajtulloch/decisiontrees      3.946s
```

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D22
[Decision Trees] MongoDB-backed trainer for decision tree tasks

Summary:
1. Claims tasks from MongoDB collection.
2. Reads data files from GridFS.
3. Publishes trained forests into same MongoDB collection.

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D23