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 .