Update the logic of solve method

[zhangt58]

To correctly handle the case of operations does not have returns.

[zhangt58]

I'm not sure what is the output if you just type pi in elegant rpn terminal, not I changed it in pyrpn, that is:

pyrpn shell > 1 pi *
pyrpn shell > 3.1415926535900001
pyrpn shell > pi
Invalid RPN, fix it and try again.:)
pyrpn shell > 

[stevemolloy]

In the elegant rpn shell:

CI0011774:~ stephenmolloy$ rpn
Welcome to rpn version 6, by Michael Borland and Robert Soliday (June 1999).
rpn> pi
  3.141592653589793

[stevemolloy]

I just looked at the updated code. Very nice. It allows cleaner implementation of functions that don't return a value.
Will you merge this into master soon?

[zhangt58]

I've updated this PR, resolved two issues:

1. Refactored solve logic to be much more clear, new function should be able to extended into Rpn very easily;
2. sto operation supported, however, the pyrpn script should be upgraded.

The feature of supporting user-defined function in CLI is still pending, that is udf of elegant rpn.

Please help to check it.

[stevemolloy]

I very much like this new structure. One worry I have is that I tend to dislike the use of getattr and hasattr methods, since they can sometimes lead to trouble -- imagine if a user-defined function shared the same name as a method of the Rpn class. But perhaps I am worrying too much :)

[zhangt58]

Good suggestion, now I make a dictionary to keep all the stored ones, pretty much simpler and clearer.