Adding expires defaultExpires function

[fishcharlie]

Summary:

This PR adds support for a defaultExpires property within the expires option.

Code sample:

Schema

 var schema = new Schema({...}, {
  expires: {
    ttl: 7*24*60*60, // 1 week in seconds
    attribute: 'ttl', // ttl will be used as the attribute name
    returnExpiredItems: true // if expired items will be returned or not (default: true)
    returnExpiredItems: true, // if expired items will be returned or not (default: true)
    defaultExpires: function () { // optional, will default to `ttl` if not defined, this function is mainly used to allow the expires to be set to null or undefined, without resetting to the normal default
      // this code will be run if `ttl` is undefined or null
      return null;
    }
  }
});

General

var item = new Item();
item.title = "Hello World";
item.expires = new Date(new Date().getTime() + (1000 * 60 * 60 * 24 * 365));
item.save(function (err, itemB) {
  itemB.expires = null;
  itemB.save(function () {
    Item.get("Hello World", function (err, itemC) {
      console.log(itemC.expires); // null

      // Normally `itemC.expires` would have been reset back to the default `ttl` which in this case is (7*24*60*60)
      // But since we override the `defaultExpires` property that function will be run since `itemB.expires` was set to null
      // And that function returns null, so it will keep it's null value.
    });
  });
});

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement
• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have confirmed that all my code changes are indented properly using 2 spaces
• I have filled out all fields above

[coveralls]

Pull Request Test Coverage Report for Build 694

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 85.186%

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Files with Coverage Reduction	New Missed Lines	%
lib/index.js	3	89.02%

Totals
Change from base Build 692:	0.03%
Covered Lines:	1881
Relevant Lines:	2140

---

💛 - Coveralls