Fix #51

- The kind() method of mesh should return a char,
like the parameter of the constructor of mesh,
not the C++ corresponding int.
- otherwise, the reduction - construction is broken
and by extension all pickling and then all mpi constructions.
Fix for clang 1y with gcc lib

- add a workaround for a gcc issue in fisrt_include.
- --> make sure that the first include is :
either a triqs lib
or first_include
Fixed antiperiodicity property of imtime

- the condition p%2 ==1 was wrong if p<0 (never true!)
- added corresponding test (gf_notail)
Added density for scalar_valued gf (and test thereof)