fix on Python2, use all __future__ imports systematically except unicode_literals, which are tricky to get right
Merge remote-tracking branch 'origin/master'
readd to_clipboard (removed by mistake I guess)
use a single method to update an axis _mapping
operations involving axes with different names do not seem like a good idea
implemented la.sum(valuegroup)
I think it's better to have add support both collections and single Axis, instead of having extend support single axes

extend's meaning need to stay close to its meaning in builtin list
(it would make append absolete)
cleanup without() code
implement del AxisCollection[axis]

simplify AxisCollection.without accordingly
added tests for AxisCollection.getitem_slice & delitem
renamed larray.py to core.py, to avoid confusion