Add -[RACSequence objectEnumerator].

As discussed in #271.

This enables this (very awkward) kind of pattern to run in constant memory:

```objc
NSEnumerator *enumerator = ...
NSEnumerator *transformedEnumerator = nil;

// References to sequences cannot be passed out of the pool
// if you want to keep the memory footprint constant
// Enumerators may be passed around without problems
@autoreleasepool {
	// Transform the enumerator using sequences
	transformedEnumerator = [[enumerator.rac_sequence map:^id(id value) {
		...
	}] filter:^BOOL(id value) {
		...
	}].objectEnumerator;
}

// Enumerate the objects and do something with them
for (;;) {
	@autoreleasepool {
		id x = [enumerator nextObject];
		if (x == nil) break;
		// Do something with x
	}
}
```