Add test/example with sparse vector features

[nevillelyh]

Sparse vectors are commonly used to extract features, and we should make sure it works seamlessly, let's at least add tests for both models (and maybe example).

[fallonchen]

Just to confirm, this means:

• Use featran to extract some data as SparseVectors, then store data as protobufs or something
• Load sparse vectors and convert them to either TFExamples or LabeledPoints for the models to use.

Is this correct?