Added force insert option when importing from array or object

`forceInsert:YES` will insert the object without checking if it's on the database first. This way we can do a lot faster imports when we know the database is empty.

Example:

    NSArray *myArrayOfDictionaries = jsonFileParsingHere...;
    [MyObject importFromArray:myArrayOfDictionaries inContext:[[LFSDataModel sharedModel] mainContext] forceInsert:YES];