Mark all euler tests as runnable

Some of them fail, as reported in Issue #135. In that case they are
marked as skip.runas
Make None<scalar> printable
Add support for exponentiation of gmp int

This fixes euler26
Work around bad indexed_ndarray assignment operator for gcc

clang is working fine, it's the same issue already encountered with
gsliced_ndarray
Fix buggy implementation of string::operator==(string_view)

It only worked when the slice had a positive step.
Fix bad implementation of char -> int conversion

Previous implementation relied on an implicit cast from char to long...
Statically type euler15, 20, 28 and 29 problems

Pythran is not capable of switching from long to big nums automatically, one
must state that big nums are to be used.
Make it possible to index strings with big ints
Have core::string use shared pointers

Necessary to fix a tricky bug with itertools.imap...

Sometimes lead to performance gain (less copies) and sometimes to
performance loss (extra indirection)

This is something we had to do anyway ;-)
Rework GMP integration

* make it possible to fall back to long long when not using -DUSE_GMP
* do not bother with manual inclusion of pythran_gmp.h, leave it to
  USE_GMP
* pythran_long_def -> pytrhan_long_t for consistency
Fix bad typing of dict.set_default

It is impossible to compute a new return type as it is incompatible with
returning a reference.
Fix bug in core::string stream unpacking

operator<< has to be in the right namespace to work (but why?)
Allow string iteration with a step !=1

Using a custom iterator, of course. As string are immutable in python,
the iterator is always a const_iterator.
Fix bug in ifilter

Previous implementation was steping its iterator one step too much.
Fix bug in assignable property propagation

It was not implemented for arrays and tuples, so tuples of GMP
expression did not compile right.
Make more euler test cases work

Sometimes, specifying that long ints are used is mandatory for pythran
compatibility.
Add missing input files for euler problem

Also fix file location for better integration with
`python setup.py test`
Merge pull request #149 from serge-sans-paille/euler-rework

Euler rework