add a three-arg version of within? in order to support non-interval types like LocalDate

This allows (within? start-date end-date test-date) for any ReadablePartials.
Merge pull request #65 from turbovote/within-readable-partials

add ability to use within? function on LocalDates
add 4 argument overlaps? to test ReadablePartials
Merge pull request #66 from turbovote/readable-partial-overlaps

add 4 argument overlaps? to test ReadablePartials
Merge pull request #68 from seancorfield/master

Fix typo in readme