Update ModelField to work with a broader range of fields

Add support for fields that use min and max value validators.
Pep8 lint
Added test for custom fields with min_value and max_value validators
Typo
Merge pull request #930 from inglesp/patch-1

Typo
Merge pull request #926 from davesque/master

I was having problems with a custom field that used a min_value validator...
Merge pull request #929 from mhsparks/positive-integer-field

Added test for custom fields with min_value and max_value validators
Added @davesque for work on #926. Thanks! :)
Merge remote-tracking branch 'upstream/master' into writable-nested-modelserializer