Use decorations API

I'm converting the gutter and highlights view to use the new decorations API.

This is good because it lets us delete a lot of functionality that's now provided by core Atom.

  - [x] Convert GutterView
  - [x] Convert HighlightsView
  - [x] Use a consolidated collection of markers
  - [ ] Update specs
In a lot of places we need to decrement a base-1 index into a base-0 index. This function does that, as well as parses as an integer and makes sure we don't end up with `-1`.