Feature/polling map

Polling Map Part 1:

Adds markers to the map at the Polling Places for the map. Map markers can be filtered with the UISegmentedControl at the top of the map view. Tapping a marker opens a popup with the location name and address.

Coming up in Part 2:
  - Tapping the popup opens a modal overlay with options to get directions to/from the polling place
  - Cache markers after use
 
TODO: Style and differentiate the different types of markers. Will do later as part of the styling tasks since we don't have images for these yet.