Refactoring of UI & Help Texts

Thomas from HockeyApp here. We invested some time to improve the Jenkins plugin. Please find the change log below:

* UI rewrite
  - Introduced radio buttons for selecting release notes type and upload methods (implemented as subviews)
  - Re-grouped configuration entries
  - Introduced warnings and errors
  - Updated help files to match our "official" wording
  - Added links to HockeyApp
* Enable overwrite of API URL to allow using the plugin with different (custom) domains
* Updated App ID handling: If an App ID is set, builds are always uploaded into this app
* Adapted plugin classes to new UI
* Implemented possibility to update from old plugin versions
* Updated Jenkins dependency to 1.509.4(LTS)