Merge pull request #5 from KostyaSha/for-merge

Portion of fixes
Allowing blank group to specify no group restrictions
Update src/main/resources/de/theit/jenkins/crowd/ErrorMessages.properties

Group is no longer mandatory
Update src/main/resources/de/theit/jenkins/crowd/ErrorMessages_de.properties

Group is no longer required
Clean-up code after cherry-picking optional functionality for group restriction.
Don't suppress compiler warnings.
Fix log messages by simplifying type.
Don't restrict groups.
More SSO optional parts.
Fix doc description.
[maven-release-plugin] prepare release crowd2-1.6
[maven-release-plugin] prepare for next development iteration
JENKINS-13279 Support for Jira embedded Crowd
Upgrade crowd integration client to 2.7.1.
Merge branch 'JENKINS-13279' of https://github.com/patbos/crowd2-plugin into patbos-JENKINS-13279
[FIXED JENKINS-21852] Added http proxy configuration.
[FIXED JENKINS-18791] Session validation interval saved from ui.
[FIXED JENKINS-13279] Don't use ssoTokenHelper, work with Embedded Crowd in Jira.
[JENKINS-16703] More options for connection configuration.
Added myself to developers.
Change developerConnection url.
Update maven-release-plugin version due to git commit issues.
[maven-release-plugin] prepare release crowd2-1.7