Fix patient builder birthdate/deathdate errors

### Story ###
- https://www.pivotaltracker.com/story/show/81387898

### Notes ###
- updated Thorax.Models.Patient.```validate()``` to utilize MomentJS versions of ```birthdate```, ```deathdate```, ```start_date```, and ```end_date``` to prevent undesired string comparisons
- updated Thorax.Views.PatientBuilder.```context()``` to check for a birthdate value before computing ```birthdatetime```

### Testing ###
1. Save a patient that is born on ```02/01/1970, 8:00AM``` and expired on ```11/04/2014, 8:00AM```.
  - ![screen shot 2014-11-13 at 1 20 18 pm](https://cloud.githubusercontent.com/assets/456952/5033912/2c4ed0a8-6b38-11e4-9279-b3e486e95514.png)
2. Start building a new patient, before entering any characteristics, toggle the deceased checkbox and then remove the date of death.
  - This should no longer populate the birthdate/time fields with 'Invalid date'.