Feature: Add Fibonacci retracement

[jjmilburn]

Just a note that there are a number of traders that attempt to use Fibonacci retracement, and including it here might make for a nice metric.

Maybe I'll try to add it myself once I dig into the codebase a bit more =).

[JPStrydom]

Would be nice to see some more signals to check for buy indicators. I'll look into it along with the moving average.

But please feel free to give it a shot yourself - If the code's solid, I'll add it to the project 8]