[MRG] Determine IsotonicRegression ``increasing`` by Pearson or Spearman corr rho

Sorry to mix a bit of docstring fix with a real PR, but:

1. Adding the missing ``increasing`` docstring to IsotonicRegression.  Missing here: http://scikit-learn.org/stable/modules/generated/sklearn.isotonic.IsotonicRegression.html#sklearn.isotonic.IsotonicRegression

2. Adding the option to use either ``scipy.stats.pearsonr`` or ``scipy.stats.spearmanr`` to estimate whether increasing should be True or False.  Tests included and passing for isotonic, though there appears to be an issue with ``sklearn.utils.tests.test_sparsefuncs.test_mean_variance_axis0`` for me at the moment when I merged to upstream earlier this morning.