Feature/conv put attribute no valueid

Implementing operation ID=30 in https://docs.google.com/spreadsheet/ccc?key=0Aj_S9VF3rt5DdEhqZHlBaGVURmhZRDY3aDRBdlpHS3c#gid=0, the one that's shaded orange.

It updates a context attribute value without having to use metadatas. Just specify the attribute name in the URL and a value in the body. Example:
PUT /ngsi10/contextEntities/Room1/attributes/temperature
```json
{
  "value": "30"
}
```