Merge pull request #2565 from change/change_org_testimonial

Adding change.org to testimonials
GBRT enhancements:
  Moved verbose output code to VerboseReporter class.
  Cosmit: logical structured code blocks
  Refactored: moved init state to method
  Added partial_fit to GradientBoosting
  Fix: is_classification not available; n_features not n_features_
  Tests
  Add ``partial_fit`` to docs
added EPSILON_FLT and EPSILON_DBL for almost equal impurity and fx value comparisons
fiddled with EPSILON_DBL
added monitor callback w/ early stopping support
fix: VerboseReporter remaining time in the case of partial_fit
added ZeroEstimator
use StackRecord as elements of stack not 5 consecutive entries
Tree: compute partition specific impurity and total impurity in criterion.children_impurity. Pass partition impurity to stack to avoid re-computation (saves some runtime).
New tree parameter: complete; specifies whether complete binary trees are grows or if a greedy branch of max_depth is grown with at most max_depth + 1 leafs.
set EPSILON_DBL to old val of 1e-7
add tests for complete
fix: use regressor in regression test
Add stack class
Handled exceptions in fit; Added tests
PriorityQueue for best-first tree growing
max_leaf_nodes instead of complete parameter
fix: test renamed and check only on nr of leafs.
tree code now supports both c-style and fortran inputs
tree code now supports both c-style and fortran inputs
updated after tree.pxd change
Tree ensemble classes don't enforce c-style inputs.