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.
**Note:** The input( 'i' ) and output( 'o' ) pins list should contain the pin nos. strictly in order of MSB...to...LSB. Also if the IC contains enable pins ,they should be preceding the data-input pins in the input list ( 'i' ).
**Note:** This method doesnt work perfectly for ICs taking in inputs like (0,0,0,1,0,0,0,0),(0,0,0,0,1,0,0,0),(0,0,0,0,0,1,0,0) etc. (eg. Priority Encoder IC 74147, IC74148) 
```
>>>t=IC_4001()
>>>p = {'i':[13,12],'o':[11]}
>>>t.truthtable(p)
   INPUTS      |OUTPUTS
   ------------|-----
     13   12   |  11 
   ------------|-----
      0    0   |   1 
      0    1   |   0 
      1    0   |   0 
      1    1   |   0 
``` 
PS: This is definitely not a elegant piece of code. So please leave your suggestions down in the comments .