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.
Use absolute import when importing cython code.
Compile only streamlinespeed with c++ and c for the rest.
Fix PEP8
Fix PEP8
Renamed resample() to set_number_of_points()
Compressed multiple cdef statements into one.
Move streamline related files to dipy.tracking
Fix setup.py
Merge pull request #382 from MarcCote/resample_cython

Cythonized version of streamlines' resample() and length() functions.