Fixes 498 - adds graphic source directory

Adds root directory for graphic assets source files and includes a
readme for the directory.
Merge pull request #510 from codeforamerica/498-adds-graphic-templates

Fixes 498 - adds graphic source directory
Updates and applies JSHint settings

- Adds all available enforcing options in JSHint rake task.
- Removes unsupported options in JSHint rake task.
- Cleans up JS according to stricter JSHint settings (with 2
exceptions: the regex in the feedback form manager and the many
statements warning in the function in the popup-manager).
- Refactors character limiter code to use event handler function.
- Removes unused code from details map manager code that was leftover
from when the nearby markers were present.
- Removes enquirejs and associated code from the popup-manager.
This was used for turning off the popups at small screen sizes,
however it had been disabled because it caused issues in IE.
Merge pull request #509 from codeforamerica/jshint-adjustments

Updates and applies JSHint settings
Fixes #414 - integrates rubocop gem

Adds rubocop gem and fixes majority of issues noted by cop.