Covariance-related functions for general AbstractArray

[yha]

This is mostly changing DenseMatrix in method signatures to AbstractMatrix, and running the tests also on a sparse array and a custom-typed array. Other changes:

• Specialized _symmetrize! for sparse matrices
• Fixed bug with zero variances in cor2cov! discovered when testing with sparse matrix
• Added mean_and_cov(vector). I didn't add mean_and_cov(vector, weights) since that would need a fix to #409, which has a long discussion pointing to JuliaLang/Statistics.jl#2

[yha]

Rebased

[yha]

bump