Add support for custom fields of type Version Picker

[Stefan Glase]

Support the custom field types:

• field name: Version Picker (single version)
  class name: com.atlassian.jira.plugin.system.customfieldtypes:version

• field name: Version Picker (multiple versions)
  class name: com.atlassian.jira.plugin.system.customfieldtypes:multiversion

The VersionManager can find the versions in the project:
https://docs.atlassian.com/software/jira/docs/api/7.6.1/com/atlassian/jira/project/version/VersionManager.html#getVersion-java.lang.Long-java.lang.String-