stackgbm 0.1.0

Improvements

• Use a standard dependency management strategy (thanks, @privefl, #1).
• The optimal parameter objects generated by cv_*() now can be passed to the params argument stackgbm() directly when bundled as a list. This change simplifies the interface and saves typing (thanks, @svm-zhang, #26).
• Rename the argument nfolds to n_folds, ncpus to n_threads in cv_*() and set the default value of n_threads to 1 (#18).
• Optimize the default parameter grid (#19).
• Add code examples for all exported functions (#27).