change null/undef folder references to empty string

fixes exception on path.join with node.js 0.10, which doesn't allow any
other args than strings.

Jira-issue: WP-980
Merge pull request #15 from aldafu/WP-980-node-0.10-compatibility

change null/undef folder references to empty string
Add another piece of configuration data: invitation tokens

These tokens are used when inviting someone to join your personal
zone for the first time, and must be persistent.

Jira issue: WP-1064
Merge pull request #16 from jplyle/support-invite-tokens

Add configuration support for invitation tokens
Merge remote-tracking branch 'upstream/master' into dashboard