Improve nt2 installation

* Remove very long installation information
* Make sure repo is updated to the last release
Merge pull request #284 from pbrunet/install

Improve nt2 installation
Remove all LocalVariable metadata as scope analysis is better
Add int specialization for None
Add new test case for unrolled list comprehension comprehension
remove_cv for pair have to be done earlier
Add documentation for part of Backend.Cxx