Cythonized resampling of streamlines
Added streamline.py in dipy/core and cythonized resample() and length()

streamline.py will contain everything related to streamlines representation.
Right now, two functions are moved there from tracking/metrics.py: resample() and length().

streamlinespeed.pyx contains the Cython part of streamline.py's functions.

setup.py has been changed to compile Cython with C++ instead of C since streamlinespeed.pyx
 uses C++'s vector to represent list of numpy array (i.e. list of streamlines).

reconst/quick_squash.pyx needed to be modified as compiling it with C++ failed. So, instead
of declaring `dtypes` as an memoryview of object, it is now declared as a memoryview of
numpy.dtype .
Added docstring and more unit tests for resample() and length().
Fixed missing parenthesis in metrics.py
Moved benchmark functions into a benchmark folder.
DOC - fix docs in tracking.utils to reflect that the affine argument is not optional
DOC - fix intro section of dipy.tracking.utils for better web rendering
DOC - move most of tracking._utils to tracking.utils so that the docs
will show up on the website
BF - add back future import
Small clean up for PR
Merge pull request #352 from MrBago/fix_utils_docs_and_affine

Fix utils docs and affine
WIP: Started implementing cross-validation from model predictions.
ENH: A bit more work on cross-validation.
BF: The order of the output needs to be like the order of the data...

Also: some more flexibility on the model API that is accepted.
PEP8

Before I start mucking about here.
PEP8
PEP8
BF: Predict properly, by separating b0s and diffusion-weighted samples.
WIP: Work towards xval prediction in csdeconv.
DOC: optional arguments and their default values.
RF: Make this part of the reconst module, instead of its own thing.