Benchmarks for operators

This uses Benchmark.jl to compare the performance of operators on
DataVectors and DataMatrices with varying numbers of NAs versus
operators on Vectors and Matrices.
Merge pull request #34 from JuliaStats/sjk/benchmark

Benchmarks for operators
Faster find for Bools
Merge pull request #33 from garborg/fasterfind

Faster find for Bools
Add .travis.yml file
Add Travis build status to README.md

This should also trigger our first build
Fix bug in Ranges DA constructor
Use Stats 0.3