RoboVM gradle plugin maintenance

RoboVM gradle plugin maintenance PR292, in it:

1. added dependency to build task to force java compilation for every RoboVM tasks. Otherwise commands like launchIOSDevice will fail as java classes are not generated.
2. added description to tasks:

   > gradle tasks
   MobiVM tasks
   ------------
   createIPA - Creates .ipa file. This is an alias for the robovmArchive task
   launchConsole - Runs a console app
   launchIOSDevice - Runs your iOS app on a connected iOS device.
   launchIPadSimulator - Runs your iOS app in the iPad simulator
   launchIPhoneSimulator - Runs your iOS app in the iPhone simulator
   robovmArchive - Compiles a binary, archives it in a format suitable for distribution and saves it to build/robovm/
   robovmInstall - Compiles a binary and installs it to build/robovm/
   

3. added logic not extracting same file on every launch and do clear cache when new files extracted just to avoid bug cases when incompatible data stays in cache, check post by @dthommes. Similar as it was done for Idea plugin.
4. added support for launching debugger, similar as @dukescript did for ‘maven plugin’:

   .. start with
   > gradle --no-daemon -i -Probovm.debug=true -Probovm.debugPort=7778 -Probovm.arch=x86_64 "-Probovm.device.name=iPhone SE" launchIPhoneSimulator
   .. attach with
   > jdb -attach 7778