Add `location` parameter to filter results in a given polygon

See #34.

The required field is created internally and removed before serving (see #33).

Deployed to staging: http://staging.api.lobid.org/oer?q=*&location=40.8,-86.6+40.8,-88.6+42.8,-88.6+42.8,-86.6

This uses spaces (you can use `+` in the request) as the point delimiters instead of the semicolons suggested in #34 since semicolons have a reserved meaning as an optional query parameter separator: http://www.w3.org/TR/1999/REC-html401-19991224/appendix/notes.html#h-B.2.2

Assigning to @literarymachine for review.