adding a few more Fibonacci algorithms

+fibonacciWithDecimal: uses NSDecimalNumber and can get up to the 186th
number

I'm new to GitHub and I don't know if I'm doing this right… So bare with me, if I screw this up. ;-)

I looked at your Fibonacci algorithm and noticed, that it does only get the numbers right up to the 92nd Fibonacci. This is because, I guess, NSNumber internally uses unsigned long long. Using NSDecimalNumber we're able to get up to 186 correct Fibonaccis. I implemented this one in +fibonacciWithDecimal.

Feel free to include this into your repo, if you think this is worth it.

Thanks for sharing your algorithms! Amazing! :-)