[MRG+1] Adding dense pipeline support to RandomTreesEmbedding by adding a return_dense argument

Hello,
  Currently, you can't use RandomTreesEmbedding within a pipeline if the following classes expect dense input.  For example, this pipeline will fail:

```
model_pipeline = sklearn.pipeline.Pipeline([
    ('scale', sklearn.preprocessing.StandardScaler()),
    ('feature_select', sklearn.feature_selection.SelectPercentile(sklearn.feature_selection.f_classif)),
    ('dim_red', sklearn.ensemble.RandomTreesEmbedding()),
    ('classify', sklearn.ensemble.ExtraTreesRegressor())
])
```

With this exception:
```
TypeError: A sparse matrix was passed, but dense data is required. Use X.toarray() to convert to a dense numpy array.
```

  To make this more convenient, I've added a return_dense argument to RandomTreesEmbedding that preserves default sklearn behavior, but allows the pipeline to behave properly with subsequent dense-expecting classes as follows:

```
model_pipeline = sklearn.pipeline.Pipeline([
    ('scale', sklearn.preprocessing.StandardScaler()),
    ('feature_select', sklearn.feature_selection.SelectPercentile(sklearn.feature_selection.f_classif)),
    ('dim_red', sklearn.ensemble.RandomTreesEmbedding(return_dense=True)),
    ('classify', sklearn.ensemble.ExtraTreesRegressor())
])
```