Add proper support for custom property exploration

[joffrey-bion]

Type exploration, as of now, is based on fields instead of actual serializable properties.
Without a more flexible approach, we end up with problems like:
#10
fabiomaffioletti/jsondoc#153
fabiomaffioletti/jsondoc#159
fabiomaffioletti/jsondoc#199
fabiomaffioletti/jsondoc#215
fabiomaffioletti/jsondoc#207
fabiomaffioletti/jsondoc#230

We want to be able to customize the way we list properties of a class, in particular how we name them and which ones to ignore.