Added Truthtable method for ICs

'Truthtable' method when applied can give the user a truthtable corresponding to the set of inputs and outputs pins specified by the user for a defined IC.
```
>>>t=IC_4001()
>>>p = {'i':[13,12],'o':[11]}
>>>t.truthtable(p)
   INPUTS      |OUTPUTS
   ------------|-----
     12   13   |  11 
   ------------|-----
      0    0   |   1 
      0    1   |   0 
      1    0   |   0 
      1    1   |   0 
```