chore(package.json): Update dependencies in package.json

Updated version of moment to include version that has a fix for the RegEx DoS defect. Also updated dev dependencies.

chore(travis): Update node to version 6 in travis.yml

feat(input): Allow user to clear input and be valid if not required

Update parsers, fomatter, and validator to handle empty values and treat them as valid.

Merge pull request #11 from dalelotts/develop

feat(model-type): Able to store date in the model as a Date, moment,milliseconds, or custom format