Implement functions for each instruction

[s-daveb]

Going hand-in-hand with issue #6, we need to implement functions that perform the actions of each instruction in valid 6502 assembly code. Since addresing mode is determined by the instruction opcodes, (found here) we will have several opcodes for each instruction, all calling the same function, but each of which will "fetch" the correct parameter using an "address mode selection" method as discussed in issue #6.

This will enable us to "generally" implement each instruction once, and to fetch its input parameter in various different ways.

[s-daveb]

Splt into two Issues: #12 and #13