A bunch of new features

Probably bad form to lump them altogether in a single pull request, but they're all in separate commits if they need to be teased apart:

- Added title to marker so that it shows up on hover
- Added description, date, & links to both DPLA record & original record to info window
- Added spiderify so that it's possible to see multiple things geocoded at the exact same spot (very frequent since DPLA geocodes quite coarsely in many cases)
- Fixed 2x problem with search diameter

I had lots of other ideas, but the data is too much of a mess right now to really make it worth investing the time.