[MRG] Sparse input support for decision tree and forest

This pull request aims to finish the work in #2984.
For reference: #655, #2848 

TODO list:

- [x] clean rebase (remove unwanted generated file, github merge commit, ...)
- [x] simpler input validation for tree base method (safe_array, check_array?)
- [x] reduce testing times and rationalize tests
- [x] use the new safe_realloc
- [x] document fit, ... with the sparse format support
- [x] fix bug in algorithm selection binary_search
- [x] raise error if int64 sparse index matrix
- [x] test oob scores
- [x] test random tree embedding with sparse input data
- [x] ensure that X is 2d
- [x] test forest apply (sparse)
- [x] test error raised if bootstrap=False and oob_score=True
- [x] test not fitted and `feature_importances_`
- [x] Check warning oob_score
- [x] Let tree estimator handle int64 based sparse matrices
- [x] switch xrange -> range
- [x] Update adaboost for dtype

Todo later:
- update what's new


If the values and indices vectors are of the right type and contiguity, the CSR constructor doesn't copy them:

```python
>>> from scipy.sparse import csr_matrix
>>> A = csr_matrix(np.arange(9).reshape(3, 3))  # to get these vectors quickly
>>> data, indices, indptr = A.data, A.indices, A.indptr
>>> A = csr_matrix((data, indices, indptr))
>>> data[:] = 1
>>> A.toarray()
array([[0, 1, 1],
       [1, 1, 1],
       [1, 1, 1]])
```