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 
Can you clarify "not feasible?"  I'm not sure I follow.

This follows the pattern of `BaseCourseEnrollmentModelSerializer`, and I thought it reasonable this class act as an extension point.  A mixin is fine too--the `BaseCourseEnrollmentModelSerializer` could be refactored to use it in that case.