[#1682] [#1681] Small fixes for docviewer project documentation

Fixed some issues with docviewer plugin in project documentation generation: 

- NPE on doc:export when no documentation folder
- Invalid zip file generated if project name has some disallowed file name characters
- Remove some unused imports

fixes for:
[#1681](http://play.lighthouseapp.com/projects/57987/tickets/1681-docviewer-cannot-generate-zip-with-in-project-name)
[#1682](http://play.lighthouseapp.com/projects/57987/tickets/1682-docviewer-npe-on-docexport)