Adds convenience subscripting for NSURLRequests.

In the spirit of making networking easier, I propose this small category that always does the right thing and makes NSURLRequest configuration far more expressive.

I originally released this into the public domain as [JLPRequestSubscripting](https://github.com/swizzlr/JLPRequestSubscripting), but it wasn't in a form where many people would use it. I realised that its goal was quite aligned with AFNetworking's: make networking in Cocoa *fun*.

While it arguably is an odd extension for the project, I'd be happy to take suggestions as to where else I could offer it.

At the least, it's worth letting you know about it.

I include the old README below, since it includes some examples of usage:

Terrific HTTP header subscripting for NSURLRequests!

The Old Way:

This returns a request with header `Accept: application/json, text/plain`:

	NSMutableURLRequest * mutableRequest = [[NSMutableURLRequest alloc] init];
	[mutableRequest setValue:@"application/json" forHTTPHeaderField:@"Accept"];
	[mutableRequest addValue:@"text/plain" forHTTPHeaderField:@"Accept"];

This returns the default value based on device locale, probably `en;q=1` if you're reading this:

	NSURLRequest * request = [[NSURLRequest alloc] init];
	[request valueForHTTPHeaderField:@"Accept-Language"];
	
	
The New Way:

Just want it with one value? Assign a string.

	NSMutableURLRequest * request = [[NSMutableURLRequest alloc] init];
	mutableRequest[@"Accept"] = @"application/json";
	
Want two? Pass in a collection.

	mutableRequest[@"Accept"] = @[@"application/json", @"text/plain"];
	
And want to get the value out?

	NSURLRequest * request = [[NSURLRequest alloc] init];
	request[@"Accept-Language"];
	
A note on assigning a collection
The collection object must conform to the `NSFastEnumeration` protocol. Freely use dictionaries, arrays or sets as the rvalue. It is left to the programmer to ensure that it only contains objects of type `NSString *`.

`JLPRequestSubscripting` will nil out any value for the current header and use fast enumeration to add values. `NSMutableURLRequest` will take care of the proper delimiting.