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 
If a mixin is not feasible--not sure if DRF supports them--this should be named something more appropriate (e.g. `ModelSerializerWithCreatedField`). The current name only makes sense if the plan is to add additional fields shared by distribution models.