Internal changes

• Fixed conflicts with Skript dev32+.
• Fixed issues #11, #12, #15, #17 and #20.
• Added support to use a list of permission in execute command with permission effect.
  Example:

  execute player command "/cmd" with permissions "first.permission" and "second.permission"

Internal changes:

• Changed documentation generator system, cleaned the code and added support for other types of files (markdown, json and yaml).
• Added an ID field for docs generator, useful for external docs tools. (Thanks to @eyesniper2)
• Fixes for gui manager system.
• Using gradle as dependency manager.
• Changed the package from me.tuke.sktuke to com.github.tukenuke.tuske.