Partial order implementation.
Corrections to partial order implementation:
- removed optional comparison operators (they are not used in the mathematical literature, and it is not necessary to burden Spire with additional operators),
- renomed doubleCompare to partialCompare,
- added laws for PartialOrder and Order.
Interval partial order.
Merge commit 'a18c5916341c2a5a7f9b1ad373566f44d16bd00b'
add richer operators for testing interval membership
Generalize symbolic set operators a bit.

Previously they were defined in Interval.scala but lived in the
spire.optiona.unicode package. Now, the implicit class has been
moved there, and a companion class for Set[A] has been added as
well.
very minor GUIDE typo
pmin, pmax syntax.
Scala wrappers ? please have a look
Merge commit 'ad44464ef50b456037e42b92b19889ad88c5006a'
Added authorship of PartialOrder.
Corrected typo in PartialOrder syntax test.
Implementation PartialOrder[Interval[A]], with semantics I < J if I is "completely on the left" of J.
Split the Ranged interval case class into Bounded, Point and Empty.
Added convenience apply and extractor to Ranged while the rest of the code is adapted.
Replaced expensive "equals" method.
Changed confusing notation: openAbove -> openUpper, openBelow -> openLower.
no message
Refactored Interval.isEmpty, Interval.isPoint.
Refactored isBelow(At), isAbove(At) and corrected a bug for empty intervals.
Refactored toString.
Refactored a few simple methods.
Added test cases.
Merge pull request #271 from dusank/topic/Fix-typos-and-imports

Fix typos, Remove unused imports