Enable date shifting a user's patient records via rake task

### Story ###
- https://www.pivotaltracker.com/story/show/63411726

### Notes ###
- added ```rake bonnie:patients:date_shift``` rake task with ```EMAIL, DIR, SECONDS, MINUTES, HOURS, DAYS, WEEKS, MONTHS, YEARS``` arguments
  - Use ```EMAIL``` to denote the user to scope the date_shift for all associated patients' source data criteria; first user by default.
  - Use ```DIR``` to denote direction [forward, backward]; direction is forward by default.
  - Use ```SECONDS, MINUTES, HOURS, DAYS, WEEKS, MONTHS, YEARS``` to denote time offset [###]; offsets are 0 by default.
- updated Patient Data Criteria model to use ```bonnie.measurePeriod``` when specifying the default dates for a dropped data criteria in the patient builder
- passed in bonnie.yml's ```measure_start_period``` into export action for the users_controller when exporting a user bundle within the admin users view
- specified an ```effective_date``` variable in the measure javascript containing bonnie.yml's ```measure_start_period``` (with computation for the end date)