Adds list/map selector

Adds two buttons above the results list for choosing a list or map view.
Adds map view space

Adds area for map view with temporary red background.
Adds results view swapping behavior

Handles showing and hiding of list and map views in the results when
associated buttons are pressed.
Adds mapview and sorting buttons

Adds map view with search results shown on mapbox map and adds buttons
for sorting (but they are not yet functional).
Adds map details link to markers

Clicking map markers sent the user to the wiki page for D.C. It now
sends them to the details page for an entry.
Adds check for existence of lat/long in results

Results that had a null value for their lat/long were causing a JS
error in mapbox. This adds conditional code to fix that problem.
Adds conditional code to check for existence of elements

JS was throwing errors when elements referenced on page were not
present. Conditional code has been added that checks for the presence
of required HTML elements. This is a symptom of a wider issue noted in
issue #111
Comments out sorting buttons

The sorting functionality needs to be addressed in a separate branch,
so the buttons in the UI have been commented out temporarily so this
map view branch can be merged, at which point another branch for
sorting behavior can be created.
Merge pull request #112 from codeforamerica/map-view

Map view