Convert NSString to NSNumber when importing

## Problem:
I am working with an app that needs to import data from a Rest API. Everything looks OK, however when importing the data I see some fields in the database that are **declared as numbers** but the server side gives them inside "", which will consider them a **string**.

## Current result:
The importation fails, and none of the data is imported

## Solution:
I implemented a new check in the ``NSAttributeDescription+MagicalDataImport`` that, in case of expecting any type of integer, checks if the data is a NSNumber, and if not it tries to convert it.
I also included the unit tests related to it. 

## Result with the fix:
Strings are converted to numbers when needed, and therefore the data importation works as expected.


I hope you find everything OK and can me merged quickly.
Thanks!