Overriding Model #56

Hi I've prepared a fix for that. Please check my branch:
https://github.com/michalig/swagger-maven-plugin

Now you can configure overridingModels in apiSource configuration:
```xml
<apiSource>
	...
	<overridingModels>/swagger-overriding-models.json</overridingModels>
</apiSource>
```
Example swagger-overriding-models.json:
```json
{
	"mapping" :
	{
		"className" : "org.joda.time.DateTime",
		"jsonString" : "{\"id\": \"Date\",\"properties\": {\"value\": {\"required\": true, \"description\": \"Date in ISO-8601 format\", \"notes\": \"Add any notes you like here\", \"type\": \"string\", \"format\": \"date-time\"}}}"
	}
}
```