[Work In Progress, WIP]: Implementation of the full range of fields stated.

I'm working towards folding in the full set of mapped fields as specified in the README.

Most of the fields use widgets that simply extend TextInput, meaning that they can share the same logic in value_from_datadict. However, fields such as BooleanField use a CheckboxInput, which requires slightly different handling. I had an idea to simply inherit a different mixin for such fields. I then created an abstract MappedWidgetMixinBase class for each specialised mixin to inherit from (identification purposes).

Also README fixes and better comments.