Introduce LambdaEqualityHelper

From the MIT licensed LibGit2Sharp project
Make ReleaseDate implement IEquatable<>
Make SemanticVersionBuildMetaData accept a ReleaseDate
Make SemanticVersionBuildMetaData implement IEquatable<>
Cover ReleaseDateFinder with tests
Make SemanticVersionPreReleaseTag implement IEquatable<>

Add mssing GetHashCode() method
Merge pull request #98 from Particular/ntk/releasedate_tests

Cover ReleaseDate underlying changes with tests