fixes typo
Don't try to generate documentation when the required options aren't set
refactoring and improved input validation for rebuildDocumentation

* replaced toAbsolutePath by pathExists / ensurePathExists and p.resolve
* throw a sensible error message when apiDocPath hasn't been set
* removed redundant variabele called 'node'
* removed unused import
Merge pull request #6 from Lutger/jsdoc

Do not try to generate documentation when the required apiDocPath option is missing