Added support for @RequestHeader

This PR adds support for another springframework annotation: @RequestHeader

The analogue annotation from jaxrs: @HeaderParam is covered in the original swagger-jaxrs version, cf:
* how it's handled in jaxrs => https://github.com/wordnik/swagger-core/blob/master/modules/swagger-jaxrs/src/main/scala/com/wordnik/swagger/jaxrs/JaxrsApiReader.scala#L102
* "header" type => https://github.com/wordnik/swagger-core/blob/master/modules/swagger-core/src/main/java/com/wordnik/swagger/core/ApiValues.java#L32

An example of mapping with @RequestHeader:
```
@RequestMapping(.....)
public ResponseEntity<MyDto> doSth(@RequestHeader("Content-Url") String contentUrl, [... other params ...]) {
    // handle
}
```

Tested against my spring-mvc controllers (3.2.2.RELEASE).