Adding a field_name to support existing entities

[MuschPusch]

Adds an option to provide a field_name to:

a.) Create fields without the field_ prefix
b.) hijack existing entities with fields which don't follow the current naming convention

Let me know what you think and I can add tests + documentation

[d34dman]

This is a nice feature :) I would like to get this in! So lets see them tests :)

[d34dman]

This can be used to enforce "re-using" of fields too. Not sure if it would increase some bugs though.