mws module: add backward-compatible support for Python 3.3+
update docs for mws's python 3 support
fix len(filter(...)) and move boto.compat import as suggested
fix has_key for python 3 compatibility
Merge pull request #2385 from felixonmars/mws-py3

mws module: add backward-compatible support for Python 3.3+. Fixes #2385.