add atomic attributes

Allows `{atomic: true}` to be set on an attribute, which triggers using
`$inc` instead of `$set` when updating the field
slight doc update
removes console statements
full test coverage
semver fix for old node
commit for travis CI
Merge pull request #13 from paulbdavis/atomic-attributes

Atomic attributes
Release 0.2.11
add detection for toMongo method

First tried to detect method `model#toMongo()` to get the document used
for saving. This allows `model#toJSON()` to be overridden to hide
hidden/unneeded fields without messing up the database document
structure.

`model#toJSON()` is used as the fallback if the model prototype does not
have a `toMongo()` method.