Properties may now be set with a hash

Instead of:

    node.set(:a, 'One')
    node.set(:b, 'Two')

You may now use:

    node.set(a: 'One', b: 'Two')

The original two argument key-value style is still supported -- and
preferred -- when you only wish to set a single property, or when you
don't want to wastefully create extra hashes.
Specify a license in the Gemspec

Closes #4.