NSURLConnection wrapper

For a dictionary like:
```
{
    @"foo" : @"bar",
    @"bar" : @baz"
}

The current implementation will return `foo=barbar=baz` while it should return `foo=bar&bar=baz`.
I think this should do the trick:

```objc
NSMutableArray *parameterStrings = [NSMutableArray array];
for (NSString *parameterKey in parameters) {
    [parameterStrings addObject:[NSString stringWithFormat:@"%@=%@", parameterKey, parameters[parameterKey]]];
}
return [parameterStrings componentsJoinedByString:@"&"];
```