Added Huffman encoding program

Described here: http://rosettacode.org/wiki/Huffman_coding
Added Huffman encoding program

Described here: http://rosettacode.org/wiki/Huffman_coding
Merge branch 'huffman-coding' of github.com:yuanchenyang/rust-rosetta into huffman-coding

Conflicts:
	src/huffman_encoding.rs
Cleaned up files and added README item
Function names use underscores, simplify Ord and Eq implementations
Merge pull request #119 from yuanchenyang/huffman-coding

Huffman coding