Fix to be more forgiving when stringifying

The implicit truthiness string for a node's `value` property
could cause problems when (albeit quite sneakily) setting
a node's `value` to an empty string.

Related bug:
  https://github.com/beaugunderson/node-sentence-tools/
  pull/4#issuecomment-66350637
Update changelog for 0.1.4
0.1.4