Feature: disable individual timestamps

[circulon]

Summary

Currently when using automatic timestamps it an all or nothing scenario on the Model.
this means that if you want a single timestamp eg craeted_at you must add the column to the migration and the model.
The use case is that some tables like log history only need a craeted attribute and this simplifies things considerably

This PR addresses the following:

• In Migrations

  • disable individual timestamps via .timestamps(created=False) or .timestamps(updated=False)
    • NOTE: if both parameters are False an Exception is thrown
  • customise the timestamp column names via .timestamps(created="created_on", updated="updated_on")

• I the Model

  • disable the individual timestamp columns via date_created_at = False or date_updated_at = False
    • this matches the migrations mechanism
    • NOTE: if __timestamps__ = True and both the date_xxx_at attributes are empty then an AttributeError is thrown notinfying the user
  • customising the column names via date_created_at = "created_on" or date_updated_at = "updated_on" then aligns with the migration

Additional:

• Fixed the Postgres Alter column ignoring the premapped_defaults if they were used generating possibly invalid sql

This also addresses the postgres alter timestamp issue in #19