add a warning to deprecate this package (in favor of MultivariateStats).

The package MultivariateStats provides better support of dimensionality reduction tasks.

See discussion in https://groups.google.com/forum/#!topic/julia-users/4ejftXei3dc.

With this PR, when this package is imported, a warning would be raised to explain the situation.

cc: @johnmyleswhite @IainNZ