[MRG] restructure and cleanup

* make absolute import the default everywhere: `from __future__ import absolute_import`
* clean up imports (reorder and group)
* rename `root_collections` to `collection` (not plural to be consistent with hist, graph, etc. and so it doesn't conflict with python's collections module)
* add `__all__` to all modules and at every subpackage level
* reduce "nestedness": `rootpy.math.physics.vector`
* remove rootpy.math after moving vector to rootpy.vector and matrix to rootpy.matrix
* rename fit to stats and move rootpy.math.stats contents to stats.utils