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

I made a cythonized version of the resampling and length functions found in `tracking/metrics.py`. As suggested by @Garyfallidis, I put these new functions in `core/streamlinespeed.pyx` accessible through `core/streamline.py`.

I made a minor change to the Makefile and setup.py to add the compilation of the new cython file. Also, regarding the language use for compiling Cython, I changed it to C++ instead of C since I needed `vector` from C++ std and also it was not breaking anything except one thing that I also fixed (see commit logs). If you don't want to change the compiler for Cython, I can set it only to compile `core/streamlinespeed.pyx` instead.
@emanuele has also a way to represent a streamline as vectors using other streamlines. This would also go to this streamline.py file in dipy.core.