Revise architecture around exporting properties

[ljacqu]

I want to rewrite how property types and exporting is handled by ConfigMe.

We avoid doing anything YAML-specific in Property itself because it should be format-agnostic, but this means that specific property types are referenced in the YAML classes. This is not portable since any other format implementation will have to do the same... What would be better is to have a new method on Property that defines the Java type of what gets exported.

[ljacqu]

One idea to make things a little faster is to keep a "register" (?) of values by property. If we validate that no properties overlap (#24) there's no reason why we couldn't read values from the YAML once and then keep everything typed somewhere, instead of going through the YML resource's map every time and performing type checks / conversion etc.

This changes behavior insofar that a bean property (or any other immutable values) would immediately be changed when an object is retrieved and altered. Right now it's necessary to again set the property to the given value.

[ljacqu]

Done within #56: on the Property interface we now have a method toExportValue() that should translate its value to simple types of String / Number, and lists/maps of the aforementioned types. More details will be provided once #56 is resolved.