* initial commit for an advanced 13th Age character/npc sheet

This pull request contains a completely new and rewritten character sheet for "13th Age". Its data structure is not compatible with the old 13th Age sheet, because it has a couple of new features, e.g.:

* most of the character stuff is in repeatable sections
* roll buttons for skill checks
* skill check ability modifier can be selected for roll
* possibility to add macros to powers and execute them with a roll button
* NPC data sheet

Since it is not compatible with the existing old 13th Age sheet, it is probably a good idea to offer the new one as additional sheet (similar to the different pathfinder versions).

A preview of the sheet can be found here: http://neovatar.github.io/roll20-character-sheets/13th_Age-neovatar/testbed/