Added setting field values using method missing

[basvodde]

Continuing the same evolution of the code. Method missing can also be used to set custom fields by checking for the = at the end of the method. For this pull request, it only checks for setting the value of existing fields.

Unfortunately, it is more code that I would like because of the parsing and converting of the method name to a field. More to come as this won't work for fields that weren't set before as the conversion of a method name to a field name won't work until the field name is known. The field name won't be known when it isn't read yet (e.g. when creating a new person). So, the next pull request will solve that problem.

[lucasmartins]

That's ok, it looks very clean. Thanks again!