Fix numpy_gexpr initialization

There was a confusion between universal references and R-value
references. Doing a raw copy is the best match I can find here.
Disable OpenMP support if libgomp is not found
Merge pull request #330 from serge-sans-paille/selective-openmp-support

Disable OpenMP support if libgomp is not found
Merge pull request #329 from serge-sans-paille/fix-numpy-gexpr

Fix numpy_gexpr initialization
Fix c_for pretty printer

* calling is_assigned on the loop node always catches the loop index
  assignment...
* reserve was added using the wrong parameter
* c_for_loop was never called and thus completely broken
* reserve implementation was buggy too

outch...
Add missing std::forward
Unrolled, recursive version of array offset computation
Fix bad combiner ordering

In one must first assign a type to the lhs before combining its type
with another, otherwise an incomplete type information is used.
Strict conformance to numpy's complex handling

std::complex and numpy complex behave in slightly different ways...
See pythran/pythonic/patch/README.rst for the motivation and choices.

Thanks to Pierrick for the patch for operator/
Merge pull request #317 from serge-sans-paille/fix-complex-multiply

Strict conformance to numpy's complex handling
Merge pull request #336 from serge-sans-paille/fix-ndarray-combiner

Fix ndarray combiner
Enable pep8 check with travis