Better last() implementation

- get rid of __getslice__ arg check, it won't work in python 3 anyway
- support __reversed__ protocol for reversible non-sequences
- it is also simpler and faster now
Get rid of old pypy xfail
Note about range() supporting slices in python 3
Better skip message for map() with multiple seqs in python 3
Updated tox and travis configs

- add python 2.6 to tox
- add python 2.6, 3.3 and pypy to travis
- added tox {posargs} pass-through