If createdAt or updatedAt is already present, ignore adding it

[luckydev]

If a schema already has createdAt field, dont add it. Instead, add updatedAt field alone and maintain it. Vice versa for the updatedAt field.

Right now in version 0.3.0, this is the error I get when createdAt is already there and if I use mongoose-timestamp in that schema.

/Users/Anand/xyz/node_modules/mongoose-timestamp/index.js:34
      .get( function () {
       ^
TypeError: Object #<Object> has no method 'get'
    at timestampsPlugin (/Users/Anand/xyz/node_modules/mongoose-timestamp/index.js:34:8)
    at Schema.plugin (/Users/Anand/xyz/node_modules/mongoose/lib/schema.js:570:3)
    at Object.<anonymous> (/Users/Anand/xyz/models/session_token.js:12:20)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/Users/Anand/xyz/models/agent.js:17:20)