:lipstick:
Publicize and document StyleManager
Clarify documentation
Deprecate style sheet events on ThemeManager

They are now supplanted by methods on the StyleManager located at
`atom.styles`.
Move public stylesheet related methods off of atom.themes

* Move atom.themes.getUserStylesheetPath to 
  atom.styles.getUserStyleSheetPath
* Deprecate atom.themes.requireStylesheet
Merge pull request #4223 from atom/ns-deprecate-style-sheet-events

Deprecate style sheet event methods on ThemeManager