Support serializing/deserializing ZonedDateTime using ToString format
Remove unused usings
Rename WithToStringZonedDateTimeSerializer to WithGeneralIsoZonedDateTimeSerializer
Move 'ToStringSerializerTests' to ExtensionsTests class.
Merge pull request #3 from tvjames/tostring-zoneddatetime-format-support

Support serializing/deserializing ZonedDateTime using ToString-ish format