Support for repository checkout directory configuration

this release adresses the declined pull request jenkinsci#6
also errors are fixed described in jenkinsci#7

PERHAPS there are difficulties when someone will delete the default „.“ setting for workFolder but will save an empty value.

But that’s a minor problem to me that might be fixed in a refactoring round.
e.g. Server and PlasticTool, one of both is obsolete to me

This release is sent to support@codicesoftware.com for merge

=> http://plasticscmsupport.zendesk.com/requests/5689

Update: seems to it is rewritten and implemented in PlasticSCM Plugin 2.2 @ jenkinsci@e598cd93 and jenkinsci@c811acc7
Unfortunatly there is no release for 2.2 atm: https://updates.jenkins-ci.org/download/plugins/plasticscm-plugin/ || https://wiki.jenkins-ci.org/display/JENKINS/PlasticSCM+Plugin