Mapper improvements

I have improved Annotation Driven Mapper with the following:
 - One annotation `@Map` instead of `@MapExactlyTo` and `@MapNested`. All `@Map `parameters are optional. By default:    
   - `to=...` parameter is set to the same name as the field annotated by `@Map`,   
   - `fromNested=...` is empty,   
   - `of=...` targets all possible classes
   
 - `TypeConverter` integration. Multiple converters are included but you can write and register your own easily.
   
 - Added support for mappable class dependencies, see: [StudentWithMappableGrade](https://github.com/jsolve/sweetener/blob/mapper-improvements/src/test/java/pl/jsolve/sweetener/mapper/stub/StudentWithMapableGrade.java?pr=%2Fjsolve%2Fsweetener%2Fpull%2F47)
    
Share your thoughts about my work. :octocat: