Added convenience method for adding multiple object mappings at once.

A convenience method for adding multiple DCObjectMappings. 

So:
```
DCObjectMapping *idMapping = [DCObjectMapping mapKeyPath:@"id" toAttribute:@"externalId" onClass:[TCBCustomer class]];
DCObjectMapping *nameMapping = [DCObjectMapping mapKeyPath:@"name" toAttribute:@"customerName" onClass:[TCBCustomer class]];
DCObjectMapping *phoneMapping = [DCObjectMapping mapKeyPath:@"phone" toAttribute:@"customerPhone" onClass:[TCBCustomer class]];
DCObjectMapping *emailMapping = [DCObjectMapping mapKeyPath:@"email" toAttribute:@"customerEmail" onClass:[TCBCustomer class]];
[config addObjectMapping:idMapping];
[config addObjectMapping:nameMapping];
[config addObjectMapping:phoneMapping];
[config addObjectMapping:emailMapping];
```
Becomes:
```
DCObjectMapping *idMapping = [DCObjectMapping mapKeyPath:@"id" toAttribute:@"externalId" onClass:[TCBCustomer class]];
DCObjectMapping *nameMapping = [DCObjectMapping mapKeyPath:@"name" toAttribute:@"customerName" onClass:[TCBCustomer class]];
DCObjectMapping *phoneMapping = [DCObjectMapping mapKeyPath:@"phone" toAttribute:@"customerPhone" onClass:[TCBCustomer class]];
DCObjectMapping *emailMapping = [DCObjectMapping mapKeyPath:@"email" toAttribute:@"customerEmail" onClass:[TCBCustomer class]];
[config addObjectMappings:@[idMapping, nameMapping, phoneMapping, emailMapping]];
```