Using the modules's name instead of modules's name.property.

Currently for config such as:
```
modules: {
        mongodb: {
            name: 'winston-mongodb',
            property: 'MongoDB'
        }
    },
```

`MongoDB` is used as the transport name.  This shouldn't be the case because:

1. `property` is an optional value so if not provided, it'll be undefined.
1. This limits you from using the same module for multiple transports.

(This also allows me to use winston-file transport twice for issue #6.)