Upgrade Scala and SBT, switch Android plugin, remove iOS plugin (for now)

This big commit updates Scala to 2.11.1 and SBT to 0.13.5. It was possible
because of new Android plugin. This template now uses
https://github.com/pfn/android-sdk-plugin plugin.

Directory layout of project was changed to mirror those of official
gdx-setup.jar, though proguard rules are different - they are
merge of rules from old project, gdx-setup.jar template rules and
official android rules from proguard-android.txt.

Proguard rules might still need some polishing, there are warnings,
but they seem not influence normal working of Libgdx, so they
are ignored for now. If something does not work due to bad proguard
rule, open ticket and it will be fixed in template.

Last, but not least, I disabled iOS code for now, due to few reasons:

  - https://github.com/ajhager/libgdx-sbt-project.g8/issues/78
  - https://github.com/roboscala/sbt-robovm/issues/10
  - default template from gdx-setup.jar looks different (it has three files)
  - I don't have iOS nor Mac to test/fix this stuff

That's why I decided to remove those files and publish what I already got,
and wait for patches from someone who has iOS or Mac and can test/fix
iOS support in new template.