Field conversion

Added the ability to have different serialized field keys from django's model field keys.

I noticed that there is a `get_field_key` function which returns field keys to the user with a modified value, but then there was no mechanism to convert user supplied data back to the django field value.  This results in the user having to know the django field names are which defeats the purpose of being able to specify different serialized keys.

I didn't see any issues mentioning this particular problem so I just went ahead and implemented it.  There are probably improvements to be made still so let me know what needs to be done.  The first of which should be documentation.