Added BaseDistributionSerializer to standardize formatting of "created" field.

This updates the formatting of the `created` field from `2014-12-05T22:31:28` to `2014-12-05T223128` (colons removed), which is how the other endpoints return this field.

@dylanrhodes @clintonb @jbau @rocha 
The mixin did not work--very sad.  I'll update the name to `ModelSerializerWithCreatedField`.