WIP: Streamlines API

This is an attempt at providing a general API to manage different streamlines file format. This is related to the [BIAP5](https://github.com/nipy/nibabel/wiki/BIAP5).

Right now, only the TRK format is supported but I can easily add support for both TCK and VTK. One difference you might found with `nibabel.trackvis` is that it loads the streamlines as they were saved. There is no `points_space` argument but instead some functions should be used to change the space where streamlines live: `change_space(streamlines, ref=affine/nifti)`, `trackvis2mrtrix(streamlines, ref=affine/nifti)`, etc.

I can't wait to know what you guys think about this. At some point, I really see the [TractConverter](https://github.com/MarcCote/tractconverter) being part of NiBabel.