AFJSONRPCProxy implements jsonrpc-NSProxy

Hi,

this is the first implementation for my own feature-request https://github.com/AFNetworking/AFJSONRPCClient/issues/2 ;)
No subclassing needed!

Usage:
``` objective-c

#import "AFJSONRPCProxy.h"
...
@protocol MyJSONRPCProtocol
- (void)sum:(NSArray *)numbers
            success:(afproxy_success_callback_t) success
            failure:(afproxy_failure_callback_t) failure;
@end
...
id<MyJSONRPCProtocol> proxy = [AFJSONRPCProxy proxyWithEndpointURL:[NSURL URLWithString:@"http://path.to/json-rpc/service/"] 
											  protocol:@protocol(MyJSONRPCProtocol)];

[proxy sum:@[@(1), @(2)]
    success:^(NSNumber *sum) {
    // ...
}   failure:^(NSError *error) {
    // ...
}];
```