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.
Before I moved it, I will add that streamlines are/will be used by the `tracking`, `segment` and `viz` modules. Which seems to me as many modules. If you plan to do bundle analysis and connectomics, they will be useful in those modules too. Your call.