DBSCAN modified to use balltree for Minkowski metrics.

Added tests confirming that balltree DBSCAN returns same results as precomputed distance metrics for Euclidean metric.
Added parameters to use NearestNeighbors instead of BallTree directly.
Removed case logic. Completed change to NearestNeighbor(). Now testing.
Changed nosetests to explicitly request algorithm='ball_tree'.
Work-around until NearestNeighbors bug is resolved.
Addressed formatting issues. Added docstrings for newest parameters.
No return before DBSCAN documentation.
Addressed formatting issues. Added docstrings for newest parameters.
No return before DBSCAN documentation.
Update to documentation in clustering.rst.
Merge branch 'dbscan_balltrees' of https://github.com/cleverless/scikit-learn into dbscan_balltrees

Conflicts:
	sklearn/cluster/tests/test_dbscan.py