[Decision Trees] Added BSON annotations and protobufs for serializing/deserializing training data

Summary:
Benchmarks show no performance hit, and this makes it a lot easier to de/reserialize
training data.

Test Plan:
unit tests.

```
PASS
BenchmarkRegressionSplitter          500           8195182 ns/op          217027 B/op       1274 allocs/op
BenchmarkRegressionSplitter-2        500           4309463 ns/op          215661 B/op       1309 allocs/op
BenchmarkRegressionSplitter-4       1000           3127843 ns/op          191823 B/op        690 allocs/op
BenchmarkRegressionSplitter-8       1000           2388040 ns/op          193786 B/op        693 allocs/op
BenchmarkRegressionSplitter-16      1000           2352783 ns/op          192669 B/op        693 allocs/op
ok      github.com/ajtulloch/decisiontrees      17.177s
```

Differential Revision: http://phabricator.tullo.ch/D15
[Decision Trees] Expresss `Example` struct as protobuf for de/reserialization

Summary: TSIA

Test Plan:
```
PASS
BenchmarkRegressionSplitter          500           8073409 ns/op          216663 B/op       1283 allocs/op
BenchmarkRegressionSplitter-2        500           4388759 ns/op          215761 B/op       1314 allocs/op
BenchmarkRegressionSplitter-4       1000           3105846 ns/op          191886 B/op        695 allocs/op
BenchmarkRegressionSplitter-8       1000           2444174 ns/op          193871 B/op        698 allocs/op
BenchmarkRegressionSplitter-16      1000           2430545 ns/op          192631 B/op        698 allocs/op
ok      github.com/ajtulloch/decisiontrees      17.274s
```

Differential Revision: http://phabricator.tullo.ch/D16
[Decision Trees] Added golang/lint to Makefile

Summary: Will improve code style in this project.

Test Plan: unit tests.

Differential Revision: http://phabricator.tullo.ch/D17
[Decision Trees] [Trainer] Basic interface for MongoTrainer

Summary: Basic code to encapsulate training based on files in GridFS

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D18