add ShouldBeUnique and ShouldNotBeUnique for collections
fix code formatting
message generator
Updated message generators and removed ShouldNotBeUnique
Merge pull request #152 from JakeGinnivan/issue-140

Issue 140
#149 Add ShouldBe/ShouldNotBe overloads for DateTime with Timespan tolerance
#149 Add ShouldBe/ShouldNotBe overloads for DateTimeOffset with Timespan tolerance
#149 Add ShouldBe/ShouldNotBe overloads for TimeSpan with tolerance
Add message generators for should methods that take tolerances