Update to marshmallow 1.0

- Update adapter to use `Schema#dump`
- Update minimum version requirement for marshmallow
Docs update: Use 'Schema' rather than 'Serializer'

...for marshmallow class names
Update requirements

NOTE: six is required for schematics 1.0
Fix py3 incompatibilities in responder.py
Add tox and travis support
Update trove classifiers
Merge pull request #6 from sloria/marshmallow-1.0-support

Update to marshmallow 1.0
Merge pull request #7 from sloria/py3

Python 3 support
Remove hard dependencies that breaks on Python 3

From now it's up to the user to decide which serialization library to
use. If you're on Python 2.* you can still use schematics, otherwise you
have to go with marshmallow with has Python 3.* support.

Closes #5.
Import Schema instead of Serializer from marshmallow

Also get rid of unnecessary serializer instantiatiions in
adapter_for
Merge pull request #8 from sloria/fix-marshmallow-import

Import Schema instead of Serializer from marshmallow