Fixes #116 - updates map markers

Updates map markers for better conformity with map clusters, and better
contrast with map appearance.

@alanjosephwilliams You def merged this earlier in https://github.com/codeforamerica/trails-mobile/pull/124 but it's not in the commit history https://github.com/codeforamerica/trails-mobile/commits/master . I think I accidentally wiped it out with a rebase. Please merge again.