Create HuffmanEncoding.cpp

What a ball-ache of an assignment. It felt very counter-intuitive -
 like bottom up design. I recommend that people attempting the 
assignment read through and note the handout 5 times before starting.
 I think some of the code I wrote is quite elegant but I got lazy with
a couple of helper fns. I realised I'm not comfortable with pointers and 
the encoding code took me longer than it should have.