Easier signal naming

Setting signal names before invoking `-logNext` was a big pain in the ass in my most recent debugging session, and led to a lot of code clutter.

This adds:

 * `-[RACStream setNameWithFormat:]`, to set a new name for a stream and still continue chaining – e.g., `[[signal setNameWithFormat:@"foo"] logNext]`.
 * ~~`RACNamedSignal()` and `RACNamedSequence()`, to declare and initialize a signal/sequence variable then give it a name.~~

I was hesitant to add more arcane macros, but it really does remove a lot of code clutter if you want the `name` property to match the variable name.