[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
- [ ] Update adaboost for dtype

Todo later:
- update what's new


You can also overwrite the output produced, so it doesn't show the whole arrays. From the docs:
```
By default, the test name output for a generated test in verbose mode will be the name of the generator function or method, followed by the args passed to the yielded callable. If you want to show a different test name, set the description attribute of the yielded callable.
```
So you can do something like
```python
check_sparse_input.description = "check_sparse_input"
```
to get rid of the verbosity.