extend methods of decimal module

[Mark H. Harris]

> 
> Have you looked at the gmpy2 ( https://code.google.com/p/gmpy/ ) module?
> 

> casevh

No...  was not aware of gmpy2... looks like a great project!   I am wondering why it would be sooo much faster?  I was hoping that Stefan Krah's C accelerator was using FFT fast fourier transforms for multiplication at least...  but, as Oscar pointed out a native python series algorithm runs right along with the built-in  (at least for exp() and ln() )   I have not looked at Krah's code, so not sure what he did to speed things up... (something more than just writing it in C I would suppose).

Thanks for the heads up on gmpy