Change RAC_DEBUG_SIGNAL_NAMES into an environment variable

I find it easier to switch an environment variable on and off from the scheme editor vs. modifying (or duplicating) a build configuration to add a new [preprocessor flag](https://github.com/ReactiveCocoa/ReactiveCocoa/pull/1086). An environment variable also avoids any need for recompilation, and can reach into dependencies.

I tested this in GitHub for Mac for a few seconds, and the total cost of `getenv()` across all calls was on the order of 20ms, so I'm not worried about it.

This also replaces some calls to `NSStringFromSelector()` in signal names with `sel_getName()`, which should be cheaper since it doesn't require `NSString` allocation. Since these are used in the argument list of `-setNameWithFormat:`, we pay the cost no matter what, so might as well shave it down a little.

/cc @joshaber