Adding custiomization options for timestamps. Closes #86

[set-killer]

I couldn't make a lot of testing but it seams to work.
Sorry, I cannot provide automatic test for this feature.

[set-killer]

@ryanfitz what do I need to complete this feature? Could you give me a hint?

This test fails because they expect type 'date' but they receive type 'alternatives', which cannot be converted to simple DynamoDB type 'DATE' . Here is example

{ id: { type: 'string', invalids: [ '' ] },
  createdAt: 
   { type: 'alternatives',
     alternatives: [ [Object], [Object], [Object] ] },
  updatedAt: 
   { type: 'alternatives',
     alternatives: [ [Object], [Object], [Object] ] } }

I think this failing test are going to be deprecated... ?