[Cython] HTML Coverage reports

[Stefan Behnel]

Hi,

I enabled HTML coverage report generation for the test suite. It's now 
available from the "HTML Coverage Reports" link of the coverage job:

https://sage.math.washington.edu:8091/hudson/job/cython-devel-tests-py27-coverage/doclinks/1/

 From a quick look, the test coverage seems not too bad and we are mostly 
lacking error tests. However, especially the branch coverage analysis hints 
at a couple of cases that are worth looking into. Compile time value 
calculation is an obvious area that is worth improving, and there are 
clearly some important untested code sections in ExprNodes.py:

https://sage.math.washington.edu:8091/hudson/job/cython-devel-tests-py27-coverage/doclinks/1/Cython_Compiler_ExprNodes.html

Stefan