Add mixin for Hash conversions

I saw that Issue #13 seems to have stalled for quite a long time.  I had some time today to put this together and would like your feedback on it, @evolve75 

Everything's organized into a new mixin, `Tree::Utils::HashConverter`.  This PR includes: 

*Class methods*
- `from_hash`, builds a `Tree::TreeNode` tree from a Ruby hash

*Instance methods*
- `add_from_hash` (somewhat similar to `add` or `<<`, but for use when information for one or more child nodes is present in a `Hash` object).  
- `to_h` hash export function. 

You'll see I also wrote a good number of assertions to verify functionality.