Timeouts for Proxy and UrlTemplate providers

Added an optional parameter "timeout" for Proxy and UrlTemplate providers.

    - timeout (optional)
        Specifies a timeout in seconds for the request (if not specified, the global default timeout
        setting will be used).

Example configuration:

```json 
{
    "name": "proxy",
    "url": "http://tile.openstreetmap.org/{Z}/{X}/{Y}.png",
    "timeout": 10
}
````